Three dimensionally periodic structural assemblies in nanometer and longer scales

ABSTRACT

This invention relates to processes for the assembly of three-dimensional structures having periodicities on the scale of optical wavelengths, and at both smaller and larger dimensions, as well as compositions and applications therefore. Invention embodiments involve the self assembly of three-dimensionally periodic arrays of spherical particles, the processing of these arrays so that both infiltration and extraction processes can occur, one or more infiltration steps for these periodic arrays, and, in some instances, extraction steps. The product articles are three-dimensionally periodic on a scale where conventional processing methods cannot be used. Articles and materials made by these processes are useful as thermoelectrics and thermionics, electrochromic display elements, low dielectric constant electronic substrate materials, electron emitters (particularly for displays), piezoelectric sensors and actuators, electrostrictive actuators, piezochromic rubbers, gas storage materials, chromatographic separation materials, catalyst support materials, photonic bandgap materials for optical circuitry, and opalescent colorants for the ultraviolet, visible, and infrared regions.

This application is a divisional of application Ser. No. 09/170,826,filed on Oct. 13, 1998, now U.S. Pat. No. 6,261,469 the entire contentsof which are hereby incorporated by reference.

GOVERNMENT STATEMENT

This invention was made with Government support under ContractDAAB07-97-C-J036 awarded by the Department of Defense. The Governmenthas certain rights in this invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to processes for the synthesis ofthree-dimensionally periodic structures and functional composites by theself-assembly of spheres, followed by one or more structuremodification, infiltration, and extraction processes. These structurescan be applied as thermoelectrics and thermionics, electrochromicdisplay elements, low dielectric constant electronic substratematerials, electron emitters (particularly for displays), piezoelectricsensors and actuators, electrostrictive actuators, piezochromic rubbers,gas storage materials, chromatographic separation materials, catalystsupport materials, photonic bandgap materials for optical circuitry, andopalescent colorants for the ultraviolet, visible, and infrared regions.

2. Description of Related Art

The art describes various means for fabricating articles with periodicstructures that repeat on the scale of millimeters, such as byconventional machining methods. Methods are also available for thefabrication of article having three-dimensional periodicities down toabout 100 microns, such as desktop manufacturing methods. On the otherextreme, three-dimensionally periodic structures with periodicities onthe scale of angstroms can be obtained by conventional crystallization.In between these extremes there exists a manufacturability gap of fromabout 100 microns to about 10 nm, where it is presently difficult orimpossible to fabricate three-dimensionally periodic structures fromdesired materials. The present invention enables the fabrication of suchperiodic structures in this manufacturability gap.

Two-dimensionally periodic structures can be created in thismanufacturability bandgap using known methods, such as optical andelectron beam lithography and mechanical embossing from lithographicallyproduced surfaces. However, achievement of similar periodicity in thethird dimension has provided the greatest problem. Limited success hasbeen achieved in creating three-dimensionally periodic structures by theself-assembly of colloidal particles (especially colloidal particlesthat are spherical and nearly monodispersed in diameter). In addition,some researchers have been successful in filling porous periodicstructures made of SiO₂ spheres with other materials, including, metalssuperconductors, and semiconductors [see V. N. Bogomolov et al. in Phys.Solid State 37, No. 11, 1874 (1995) and inPhys. Solid State 39, No. 11,341 (1997)]. However, methods have not been discovered for theelimination of the SiO₂ spheres from the infiltrated structure, and thepresence of these spheres can degrade the desired properties resultingfrom the infiltrated materials. Devising an overall process thatpreserves the structure of the three-dimensional array of infiltratedmaterial, while at the same time enabling the extraction of the SiO₂spheres, represents a higher level of difficulty which has not beenaddressed by the prior art.

The lack of more success in prior research reflects several genericissues. In order to conduct high temperature infiltration processes, itis necessary to use a first matrix material (such as an array ofcrystallized SiO₂ spheres) that is thermally and mechanically stable toabove 300° C. However, extraction processes have not been successfullydemonstrated for such thermally stable matrix materials. One reason isthat it is topologically impossible to extract such matrix materials(unless the pre-extraction processes of this invention areutilized)—because the spheres of the matrix material are buried in theinfiltrated material. Even if this topological problem could be solved,the unsolved problem still remains of conducting such extraction of ahigh-thermal-stability matrix material (like SiO₂) without disruptingthe structure of the infiltrated material. There has, however, been somesuccess in crystallizing low thermal stability polymers as matrixmaterials (which transform to a gas on heating), infiltrating thesematerials by low temperature processes, and then removing the originalpolymer particles by gas phase processes (resulting from polymerdegradation). Specifically, Velev et al. [Nature 389, 447 (1997)] madethree-dimensionally periodic shells of silica by using a chemicalreaction to form the silica as a coating within polystyrene latexparticle arrays, and then burning away the polystyrene (causing 20-35%shrinkage of the unit-cell parameter). Likewise, Wijnhoven and Vos(Science 281, 802 (1998)) made analogous crystals consisting of titaniaby assembling polystyrene latex spheres into a face-centered-cubicstructure, chemically reacting tetrapropoxy-titane inside thepolystyrene sphere structure (using up to eight penetration, reaction,and drying steps), and then burning away the polystyrene spheres(providing 33% shrinkage of the unit-cell parameter). A quite similarpolystyrene-sphere-based method was used by B. T. Holland et al.[Science 281, 538 (1998)] to make titania, zirconia, and alumina. Suchprocesses can avoid the above topological problem by using holes in thereacting coating layer (or layer permeability) to permit release of thegases produced by pyrolysis. However, this approach is generallyunsatisfactory because of (1) inapplicability for materials that aremost desirably infiltrated at high temperatures, (2) the difficulty ofcrystallizing the polymer spheres into well-ordered crystals havinglarge dimensions, (3) the possible introduction of holes in thestructure of the infiltrated material during gas evolution, (4) theoccurrence of about 20-35% shrinkage of lattice parameter of the finalstructure relative to the initial structure, which can disruptstructural perfection, (5) inaccurate replication of the void space inthe original structure (evident from the micrographs of the abovereferences), (6) the lack of mechanical robustness of the polymer sphereassemblies (which again restricts the infiltration process), (7) theimpossibility of obtaining complete filling of the void space of theoriginal opal structure by the demonstrated chemical methods (so toobtain the volumetric inverse of the opal structure), and (8) theunsuitability of template removal by pyrolysis for the preparation oflattice structures comprised of thermally labile materials, such aspolymers. As an alternative method, Imhof and Pine [Nature 389, 948(1997)] have made periodic foams by using a sol-gel process to depositmaterials in a self-assembly of monodispersed emulsion droplets.Barriers to application are provided by the lack of generality of thismethod, present inability to provide well-ordered materials of largedimensions by emulsion self-organization, the poor degree of order ofthe resulting product, and the large materials shrinkage during thedrying step for the gel (about 50%).

What is needed and what the prior art has not provided is a means forforming three-dimensionally periodic structures with periodicities onthe scale of 100 microns to 10 nm from arbitrarily chosen materials.Such materials are needed for a host of applications where the scale ofthe lattice periodicity profoundly effects properties. Moreover, theprior art has not demonstrated the ability to create the complicated,multicomponent structures needed for advanced device applications. Theformation of these multicomponent structures requires the ability toconduct multiple infiltration and extraction steps without substantiallydegrading regularity, the ability to control structural channeldimensions independent of unit-cell dimensions, the ability to conductinfiltrations at high temperatures, and the ability to controllablyengineer breaks in the continuity of infiltrated materials by melt phaseprocesses—none of which have been demonstrated by prior art processesleading to either a opal replica structure or a more complicatedstructure. Also, methods are needed for the creation ofthree-dimensionally periodic nanoscale structures with less than 26%volume filling from thermally unstable materials, such as organicpolymers (and especially elastomeric polymers and piezoelectricpolymers), and such methods do not exist in the prior art. In addition,there are no available methods in the prior art for making a materialthat is a fully filled volumetric inverse of the void space of an opalstructure, and materials with such structures are required for theapplications described herein.

SUMMARY OF INVENTION

The invention provides a process for the formation of athree-dimensionally-periodic porous structure, comprising the steps of

(a) crystallizing spheres of material A into a first structure havingthree-dimensional periodicity, and voids between spheres, wherein thematerial A is mechanically and thermally stable to at least about 300°C.,

(b) treating this first structure so that necks are formed between thespheres of material A,

(c) infiltrating said first structure with material B to form a A-Bcomposite structure, and

(d) removing material A from said A-B composite structure to form asecond structure comprising material B.

The invention also provides a process for the formation of a structurehaving three-dimensional periodicity comprising a composite material Aand an organic polymer B, comprising the steps of

(a) crystallizing particles of material A into a first structure havingthree-dimensional periodicity and lattice repeat dimensions of fromabout 20 nm to about 100 μm,

(b) infiltrating said first structure with either material B or aprecursor thereof to form a A-B composite structure.

The invention further provides a process for the formation of a porousstructure having three-dimensional periodicity comprising materials Aand B, which comprises the steps of

(a) crystallizing particles of material A into a first structure havingthree-dimensional periodicity,

(b) treating the particles of material A so that interparticle necks areformed,

(c) infiltrating said first structure with material B to form an A-Bcomposite structure, and

(d) partially or completely melting and solidifying either component Aor B, but not both.

The invention still further provides a three-dimensionally periodicthermoelectric or thermionic composition containing surfaces orinterfaces that are inverse replicas of the surfaces of a sphere array,wherein the sphere diameter is from about 20 nm to about 10 μm and thethermoelectric composition contains less than about 50 percent by volumeof an electrically insulating composition.

The invention also provides a three-dimensionally-periodic piezoelectricceramic, piezoelectric polymer, or electrostrictive material compositioncontaining surfaces or interfaces that are inverse replicas of thesurfaces of a sphere array, wherein the sphere diameter is from about 20nm to about 100 μm and wherein an obtainable electrically generatedstrain is at least 1% for the electrostrictive composition.

The invention further provides a three-dimensionally-periodicelectrically insulating structure containing surfaces or interfaces thatare inverse replicas of the surfaces of a sphere array, wherein necksexists between neighboring spheres in said sphere array and the averagesphere diameter does not exceed about 100 nm.

The invention still further provides a colorant for ultraviolet, visibleor infrared wavelengths comprising a three-dimensionally-periodicstructure containing surfaces or interfaces that are inverse replicas ofthe surfaces of a sphere array, wherein necks exists between neighboringspheres in said sphere array and the average sphere diameter is fromabout 20 nm to about 1 μm.

The invention yet further provides a process for substantiallyeliminating the coloration of a material comprising particles of acolorant in a matrix polymer wherein the particles of the colorantcomprise an array which is three dimensionally periodic at visiblewavelengths which process comprises heating the material to atemperature that is higher than the melting temperature of the colorant,and below the degradation temperature of the matrix polymer.

The invention still further provides an elastomer having athree-dimensionally-periodic structure that contains either (a) surfacesor interfaces that are inverse replicas of the surfaces of a spherearray or (b) elastomer spheres, wherein the sphere diameter is fromabout 20 nm to about 100 μm.

The invention also provides a periodic material comprising either aconducting form of diamond, diamond with hydrogenated surfaces,polycrystalline diamond where sp² carbons at grain boundaries conferelectrical conductivity, diamond-like carbon, or nitrogen-doped diamond,wherein said periodic material contains surfaces or interfaces that areinverse replicas of the surfaces of a sphere array having a spherediameter that is from about 20 nm to about 100 μm.

The invention also provides a three-dimensionally periodic materialcomprising graphite whose surfaces or interfaces are inverse replicas ofthe surfaces of a sphere array, wherein the sphere diameter is fromabout 20 nm to about 100 μm, wherein the sheets of said graphite areoriented with respect to the original surfaces of the spheres in saidsphere array.

The invention also provides a three-dimensionally periodic materialcomprising carbon, whose surfaces or interfaces are inverse replicas ofthe surfaces of a sphere array, wherein the sphere diameter is fromabout 20 to about 100 μm, and the carbon is a foam having an averagepore diameter of from about 4 Å to about 10 Å.

The invention also provides a three-dimensionally periodic materialcomprising at least three spatially separated compositions, wherein theinterfaces between these compositions are inverse replicas of thesurfaces of a sphere array, wherein the sphere diameter is from about 20nm to about 100 μm.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more clearly understood and further applicationswill be apparent when reference is made to the following detaileddescription of preferred embodiments of the invention and theaccompanying drawings, in which:

FIG. 1 is a scanning electron micrograph of porous SiO₂ opals used astemplates in some of the invention embodiments. The sphere diameter is250 nm. These opals have been sintered sufficiently to generate necksbetween spheres, but not so completely as to close the pore volumeinterconnections that are required for infiltration.

FIG. 2 is a schematic representation of the interconnected network ofoctahedral and tetrahedral motifs that are filled by the infiltration ofthe material B (which is an intermediate step in the formation of avolumetrically-templated inverse opal). The gaps between these motifs inthis figure are filled by the initial infiltration process, but can beopened by the low-pressure melting/resolidification process demonstratedin invention embodiments.

FIG. 3 is a structural model for a volumetrically-templated inverse opalproduced from a face-centered-cubic opal. The large holes in thestructure correspond to sphere locations in the original sphere array ofa porous opal.

FIG. 4 is a scanning electron micrograph of a (111) plane of avolumetrically-templated phenolic inverse opal made by the processembodiment of Example 1, which uses the opal type shown in FIG. 1. Theholes in the spherical shells (left by the extraction of the SiO₂spheres of the porous opal template) correspond to the necks between theoriginal SiO₂ spheres, which enabled the extraction process.

FIG. 5 is a scanning electron micrograph of a (111) plane of a glassycarbon inverse opal resulting from the pyrolysis of a phenolic filledSiO₂ opal, followed by SiO₂ extraction (see Example 7).

FIG. 6 is a scanning electron micrograph of a (100) plane of a graphiteinverse opal made by the CVD process embodiment of Example 10. The whitesphere in the upper part of the pictured fracture surface is a SiO₂sphere which was not extracted by the hydrofluoric acid treatment(probably because this sphere was not correctly connected byinter-sphere necks to neighboring spheres—so the acid had no access tothis sphere).

FIG. 7 is a scanning electron micrograph showing a (111) plane of thediamond inverse opal made by the process embodiment of Example 13.

FIG. 8 is scanning electron micrograph showing a (100) surface of agraphite inverse opal made by the process embodiment of Example 13,which uses plasma enhanced CVD.

FIG. 9, for comparison with FIG. 8, is a model showing a (100) surfaceof a surface-templated inverse opal. The occurrence of fracture acrosssphere diameters, rather than at intersphere contact points, explainsthe difference between the FIG. 8 micrograph and this model.

FIG. 10 is transmission electron micrograph showing a surface-templatedgraphite inverse opal having a wall thickness of only about 40 Å. Thismaterial was made by the process embodiment of Example 13, which usesplasma enhanced CVD.

FIG. 11 is a scanning electron micrograph showing a (111) surface of avolumetrically templated polystyrene inverse opal, which was made by theprocess embodiment of Example 4.

FIG. 12 is a scanning electron micrograph showing a (111) fracturesurface of a phenolic-derived, volumetrically-templated carbon inverseopal, which was filled during a second infiltration/polymerization stepwith polystyrene (see Example 12). The white appearing structurecomponent is the polystyrene. The darker surrounding material is theglassy carbon of the inverse opal.

FIG. 13 is a scanning electron micrograph of a metal/glassy-carboncomposite (made by the process embodiment of Example 16) that consistsof a phenolic-derived glassy-carbon inverse opal whose network oninterconnected spherical voids are filled with a metal alloy. Anaccelerating voltage of 20 kV was chosen for this micrograph so that thepenetration depth of the electron beam in carbon was longer than thewall thickness for the carbon matrix. These walls are thensemi-transparent to the electrons, which enables the underlying layer ofordered metal spheres to be clearly seen.

FIG. 14 is a scanning electron micrograph of a bismuth inverse opal madeby the process embodiment of Example 17.

DETAILED DESCRIPTION OF THE INVENTION

The invention provides processes for the fabrication of structures thatare three-dimensionally periodic on the scale where conventionalmanufacturing processes cannot be practiced. These structures are usefulfor a variety of applications because of the functionality provided bythis periodicity. These applications include use as thermoelectrics andthermionics, electrochronic display elements, low dielectric constantelectronic substrate materials, electron emitters (particularly fordisplays), piezoelectric sensors and actuators, electrostrictiveactuators, piezochromic rubbers, gas storage materials, chromatographicseparation materials, catalyst support materials, photonic bandgapmaterials for optical circuitry, and opalescent colorants for theultraviolet, visible, and infrared regions.

A process that provides a preferred invention embodiment comprises thefollowing basic steps. (I) The first step comprises assembling nearlymonodispersed spheres of material A (such as 250 nm SiO₂ spheres) intoan “opal-like” lattice. We use this term opal-like (or opal or opaltemplate) to designate structures having a packing or spheres that issimilar to the well-known packing of SiO₂ spheres in natural gem opal.In a preferred embodiment, such assembly is by a sedimentation process(whose rate can be enhanced by centrifugation) from a colloidal solutionof the spheres.

(II) In a second step, the spheres are partially joined together so thatfinite diameter necks connect neighboring particles, but the void spacein the opal-like structure remains percolated. In a preferredembodiment, this partial joining of neighboring spheres is by sinteringat high temperatures (which are preferably between 650 and 900° C. forSiO₂ spheres).

(III) This partially sintered porous opal crystal is then used as atemplate for obtaining a three-dimensionally periodic assembly of asecond material (material B), which is infiltrated into the opal duringthe third step. This B material can be either infiltrated into the opalso as to substantially fill the void space of the opal (which is calledvolumetric filling) or the B material can be infiltrated so as toprovide only a coating on the interior surfaces of the opal (which iscalled surface filling). By infiltrated material we mean a material thatis infiltrated either in a final or a precursor state.

(IV) In the fourth step the initial opal material is dissolved orotherwise removed to obtain a hollow structure that is an inversereplica of the original opal structure. This inverse replica is eithercalled a volumetrically-templated inverse opal (FIG. 3), if the stepthree infiltration of B was volumetric templating, or asurface-templated inverse opal (FIG. 9), if the step three infiltrationof B was surface templating.

For the purposes of this invention, the following definitions are used.The primary opal template (or original opal) is the initially formedperiodic structure (from material A spheres) that has the desiredlattice periodicity. In the most preferred embodiments, this primaryopal template will be a porous lattice of SiO₂ spheres that are packedinto a face-centered-cubic (fcc) lattice. These and structurally relatedderivative structures having a structure analogous to that of naturalopal will be referred to as opals or opal templates independent ofwhether or not the spheres in the parent template are of SiO₂. Also, thestructures obtained by partial or complete filling of the opal voidspace with a second material, followed by removal of the original opallattice material, will be called inverse opals (independent of thechemical composition of the primary opal template).

In other invention embodiments this inverse opal replica is furthertransformed by additional processing. For example, thevolumetrically-templated inverse opal is used as a template for theperiodic deposition of a material C within the interstitial space leftby the removal of the original spheres of material A. In anotheroptional step, the material B can be removed, so as to result in adirect opal lattice comprising only material C. More complicatedassembly processes can be accomplished for a surface-templated inverseopal of material B, since such inverse opal has two separate void spacesthat are separately percolated, corresponding to the interiors andexteriors to the shell structure of the surface-templated inverse opal.Hence, these separate void spaces can be separately filled with eitherthe same or different materials, called C₁ and C₂. Furthermore, the C₁and C₂ materials can be infiltrated to provide either surface filling orvolumetric filling. Additionally, a surface templating step withmaterial D₁ can be followed sequentially by either surface filling inthe same void channels by one or more other materials (D₂, D₃, and etc.)or by a volume filling process with a material D₂ in the same voidchannel. In addition partial or complete removal processes can be usedfor materials modification. For example, the spheres of the primary opaltemplate can be of a material A₁ that is coated with a material A₂.After assembly of the A₁/A₂ composite spheres into a porous opal andinfiltration of the pore space in this opal with material B, thematerial A₂ can be selectively removed, so as to provide spheres ofmaterial A₁ that are free to move in the matrix of material B.

The above step II of generating an extended interface between spheres iscritical for producing inverse opals if the infiltrated materialcompletely covers the internal surface of the A-material spheres in theprimary opal template. This extended interface between spheres is mostpreferably generated by heating the opal-like sphere array to atemperature that is sufficiently high that sintering creates anintersphere neck between neighboring spheres. The temperature and timesrequired for this sintering will depend upon the composition of thespheres, shorter times and higher temperatures typically beingequivalent to longer times at lower temperatures. Such sintering processshould preferably be accomplished at below the temperature at which thespheres become sufficiently fluid that a nearly spherical shape can notbe maintained. To illustrate this approach, consider the embodiment ofthis invention where the A-material spheres are SiO₂ spheres in whichthe required interface is created by sintering. Such sintering isnormally achieved by a two step annealing process. The first step is alow temperature anneal at typically 100-120° C. for 2-3 days toeliminate excess water, and the second step is typically a hightemperature anneal at 650-800° C. for several hours (resulting inSi—O—Si linkages between the silica of two adjacent spheres). The flowof silica between spheres at high temperatures creates the necks offinite diameter (up to 125 nm for 250 nm spheres) without substantiallychanging the spherical shape of each ball. While sintering provides theinter-sphere interface through which the SiO₂ can be removed afterinfiltration, too much sintering prohibits infiltration by closing thepassages between the octahedral and tetrahedral voids in the originalfcc silica lattice. If the sintering provides SiO₂ spheres of diameterD=250 nm, which are separated by a shorter distance d, geometricalcalculations indicate the narrow range of D/d that is compatible withboth the infiltration and extraction processes. The narrowest channelconstriction between octahedral and tetrahedral void spaces occurs onthe (111) plane midway between six surrounding spheres, and has adiameter Δ=2(3)^(−½)d−D (which is 38 nm if D/d is unity and zero if D/dis above 1.155). With increasing D/d in this range, the diameter of thecircular interface between nearest neighbor spheres δ=(D²−d²)^(½)increases from zero to δ=125 nm. Infiltration and extraction processescan proceed through these narrow channels over millimeter opalthickness, thereby replicating periodic structural features of theoriginal opal crystals (the primary opal template) down to the 10 nmscale.

For the purposes of this invention, the interparticle necks betweenspheres A need not be comprised of the material A. The guideline for thenature of these necks is that they should enable the formation of acontinuous path for the extraction of material A from the surroundingmaterial B. For example, such necks can be formed by infiltrating thematerial B in the lattice of A spheres so that the inter-spherejunctions are not completely filled with material B. Alternately,intersphere junctions suitable for the purposes of this invention can beformed by surface coating internal surface of the self-assembled arrayof A spheres with a material A*, which preferably can be removed by thesame extraction process that removes the A spheres (after theinfiltration of the material B. However, the most preferred method forforming the inter-particle necks between A particles is by sintering.Pressure, preferably hydrostatic pressure at elevated temperatures, canbe used for forming mechanical deformation that leads to theseinterparticle necks. Moreover, the mechanical deformation caused by theapplication of non-hydrostatic mechanical stress (preferably at elevatedtemperatures) can be used to decrease the symmetry of the opal orderivative structures.

Depending upon the structure needed for a particular application, theprimary opal template can be exposed to a chemical that alters thesurface energy or structure of this opal prior to the infiltration ofmaterial B. For example, the surface of SiO₂ spheres can be madehydrophobic by reacting an organosilane (such as vinyltriethoxysilaneand vinyltrichlorosilane) on this surface or by infiltrating a solutionof a solid hydrocarbon (such as poly(o-phenylene)) into the opal,followed by evaporation of the solvent. Various methods can be employedfor structural modification. For example, the selective dissolution ofthe opal spheres at non-neck regions of the spheres can be used toincrease opal void volume without destroying the neck-generatedinterconnections that are required for the extraction of the A materialafter the infiltration of the B material. In this case, the partialdissolution of sintered SiO₂ opals with aqueous KOH reduces the spherediameter (thereby increasing the void volume that is available for theinfiltration of material B) without eliminating the interconnectsresulting from sintering-generated intersphere necks.

The material A is preferably mechanically and thermally stable to atleast about 300° C., more preferably at least about 500° C., and mostpreferably at least about 800° C. By mechanical and thermal stability tothese temperatures we mean that these particles have sufficientstability at these temperature to largely maintain the structuralregularity of opals based on these particles. By mechanical and thermalstability at the indicated temperature it is meant that degradationprocesses occurring at this temperature leaves a stable product whichwill not disappear by further gas evolution or fluidization at thespecified temperature. Since the function of material A is to provideself-assembly of a crystal structure; to enable the infiltration ofmaterial B under desired conditions of temperature, pressure, and othersurrounding conditions; and then to be partially or completelyremovable, these functions lead to the choice of material A. Material Ais most preferably monodispersed SiO₂ spheres having a diameter of about20 nm to about 100 μm. However, these A materials can be virtually anyorganic or inorganic composition that satisfies the above stabilityconditions. Examples are ceramics, salts, metals, carbon phases, andhigh thermal stability organic polymers.

Examples of preferred sphere types that are commercially available (forexample, from Polysciences, Inc., 400 Valley Road, Warrington, Pa.18976) are poly(tetrafluoroethylene) spheres (35 μm),poly(tetrafluoroethylene-propylene) spheres (10 μm), poly(vinylchloride) spheres (300-400 μm), phenolic spheres (hollow, 5-127 μm),graphitized carbon particles (0.027-0.030 μm), colloidal gold particles,glass spheres (various sizes from 3 to 750 μm), hollow glass spheres (2to 20 μm), and silica microspheres (0.05 μm to 0.45 μm). Some of thesecommercially available spheres are too polydispersed in diameter fordirect self-assembly into opal type of structures. Hence, conventionalmethods of size separation should preferably be used prior to theself-assembly of the spheres into an opal lattice (such assedimentation, repeated-self assembly, and sieving). Such methods forthe separation of spheres according to size are described by A. J.Gaskin and P. J. Darragh in U.S. Pat. No. 3,497,367 (1970) which isincorporated herein by reference.

Also, some of these commercially available spheres are preferablyconverted into a more stable form prior to usage (such as by thepyrolysis of poly(vinyl chloride) or phenolic spheres to make carbonspheres). Likewise, the poly(vinyl chloride) spheres andpoly(tetrafluoroethylene) spheres can be converted to carbon spheresusing well known dehydrohalogenation and dehalogenation agents (such asalkali metals) prior to their use for forming the opal matrices. Inaddition, these spheres can be coated with materials that facilitate theself-assembly process, such as organosilanes for the glass spheres. Auseful method for forming nearly monodispersed polymer spheres withdiameters in desired range (using an aerosol generator), and ofcarbonizing such spheres of poly(furfuryl alcohol) or poly(vinylacetate) to obtain carbon spheres having diameters in the range 20-150μm, are described by K. M. Daer and Y. A. Levendis in Journal AppliedPolymer Science 45, 2061 (1992). Such methods are useful for preparingspheres that self assemble into opals, which can be used for embodimentsof the present invention. Other references that describe the preparationof colloidal particles that are useful for the practice of the presentinvention embodiments are E. Matijevic, Materials Research Bulletin, pp.18-20 (December 1989), T. Sugimoto, Materials Research Bulletin, pp.23-28 (December 1989), F. Fievet et al., Materials Research Bulletin,pp. 29-34 (December 1989), A. J. I. Ward and S. E. Friberg, MaterialsResearch Bulletin, pp. 41-46 (December 1989), and J. H. Fender and F. C.Meldrum, Advanced Materials 7, 607 (1995).

For the purposes of this invention the term sphere includes nearspherical particles that pack similarly to spheres (such as oblong oricosohedral particles). Various methods well known in the art can beused to assemble the A-material spheres of the primary opal templateused for the processes of the present invention. The most preferredmethod is by the slow sedimentation of a colloidal suspension of nearlymonodispersed spheres. These spheres preferably differ in radii by nomore than a factor of 1.05. This crystallization can be optionallyaccelerated by the application of an electric or magnetic field or fieldgradient, or by using a centrifuge. Also, in order to accelerate thecrystallization process, one can optionally use a low viscosity fluid asthe suspension medium for the colloidal particles, such as supercriticalwater or carbon dioxide. For sphere sizes that are large, typicallyabove 10 μm, such sphere assembly can optionally be accomplished by themechanical vibration of either a dry or wet assembly of spheres. Asexamples of such methods, the porous silica opals used as templates canbe made by published methods used by the jewelry industry to fabricategem quality synthetic opal. For descriptions of these methods see A. P.Philipse, J. Mater. Sci. Lett. 8, 1371 (1987); H. Miguez et al. Appl.Phys. Lett. 71, 1148 (1997); C. López, L. Miguez, L. Vázquez, F.Meseguer, R. Mayoral, and M. Ocaña, Superlattices and Microstructures22, 399 (1997); N. D. Deniskina, D. V. Kalinin, and L. K. Kazantseva,Gem Quality Opals, Their Synthesis and Natural Genesis (Nauka,Novosibirsk, 1988) [in Russian]; A. J. Gaskin and P. J. Darragh, U.S.Pat. No. 3,497,367 (1970); and E. G. Acker and M. E. Winyall, U.S. Pat.No. 4,049,781 (1977) which are incorporated herein by reference. Becauseof low synthesis cost, the SiO₂ spheres are preferably formed by asodium silicate route, such as described by A. J. Gaskin and P. J.Darragh (U.S. Pat. No. 3,497,367 (1970)) and by E. G. Acker and M. E.Winyall (U.S. Pat. No. 4,049,781 (1977)). Spheres made by this processare commercially available from Nissan Chemical Company (Nissan ChemicalAmerica Corporation, Houston Branch, 12330 Bay Area Bvd., Pasadena, Tx.77507). Alternately, the SiO₂ spheres can be made by the Stöber method(A. K. Van Helden, J. W. Kansen, and A. Vrij, J. Colloid InterfaceScience 81, 354 (1981)), which results in more monodispersed silicaspheres than does the sodium silicate route. Since the sodium silicateprocess results in spheres that are polydispersed in sphere diameter,they must be fractionated according to size by repeatedsedimentation/redispersion, as described by A. J. Gaskin and P. J.Darragh (U.S. Pat. No. 3,497,367 (1970)). The growth of SiO₂ opals canbe conveniently accomplished by slow crystallization of themonodispersed aqueous colloid by sedimentation at ambient (for example,over a 10 month period in a meter-long glass cylinder), or by fastermethods described by J. Gaskin and P. J. Darragh (U.S. Pat. No.3,497,367 (1970)).

Post-crystallization sintering may be by thermal annealing, first at100-120° C. for several days, followed by high temperature annealing(for typically several hours at 750-800° C.) to produce a D/d of about1.035-1.055. Mechanical strength measurements indicated when thesintering is sufficient for the development of the required inter-sphereinterfaces. Typical resulting mechanical properties in compression forpolycrystalline centimeter-size cubes of sintered 250 nm diameter opalsspheres are a modulus of 24 GPa, an ultimate strength of 107 MPa, and afailure strain of 0.8%. The absence of over sintering (leading to voidinterconnect closure) may be insured by observing the transformationfrom the initial chalk-like appearance to a transparent, opalescentappearance when the opal is submersed in a liquid having about the samerefractive index as the Sio₂ spheres.

The most preferred structure for the primary opal template isface-centered arrangement of monodispersed spheres. Such preferredstructures usually contain stacking faults in which hexagonal interlayerpacking is admixed with cubic interlayer stacking. Other preferredstructures for the primary opal template are hexagonal-close-packedarrangements of monodispersed spheres, body-centered-cubic arrangementsof monodispersed spheres, crystalline structures comprising mixtures ofrods and spheres having monodispersed diameters, and crystallinearrangements of two or more sphere types. In the latter category, cubicstructures comprising two monodispersed sphere types N and M are morepreferred (where N are the smaller radius spheres and M are the largerradius spheres). Such structures of the MN₁₃ and the MN₂ type are mostpreferred. The NM₁₃ structure consists of twelve small spheres arrangedicosahedrally around a larger sphere, while the MN₂ structure consistsof a close-packed-layers of the large spheres stacked directly aboveeach other, and the smaller spheres located interstitially. Methods forthe preparation of these arrangements of spheres are described by P.Barrlett et al. in J. Chem. Physics. 93, 1299 (1990) and in Phys. Rev.Lett. 68, 3801 (1992). These methods are analogous to those used for thecrystallization of monodispersed spheres—the major difference beingthese phases result from the crystallization of a mixture of twodifferent radius spheres (having a radius ratio of about 0.58). Forcertain applications it is preferred that cubic or hexagonal arrays, ormixtures thereof, are mechanically deformed before or after theinfiltration of material B. The benefit of such mechanical deformationis that the void volume can be decreased for the uninfiltrated opal (andsymmetry can be decreased for either infiltrated or uninfiltratedopals).

Unless otherwise indicated here the preferred range for the diameter ofthe spheres in the primary opal template is from about 20 nm to about100 μm. However, for particular types of materials applications asmaller range of sphere diameters is preferred. This range is from about20 μm to about 10 μm for thermoelectric or thermionic applications, fromabout 20 nm to about 0.1 μm for application as low dielectric constantlayers, and from about 50 nm to about 1 μm for preferred colorants forthe visible, infrared, and ultraviolet wavelengths.

The selection of the above materials (B, C, etc.) depends upon theapplication need. For example, these materials can include opticalmaterials (such as diamond), superconductors, redox materials (such asconducting organic polymers), piezoelectrics, metals, semiconductors,thermoelectrics, electrostrictive materials, magnetoresistive, andferromagnetic materials. More specific guidance on the selection ofthese compositions will be provided latter, after we discuss theapplications needs for the structures made by invention embodiments.Materials that provide a self-assembling microstructure, such as blockcopolymers and liquid crystal compositions, are also preferred—sincethese materials can form useful sub-structures within the opals and opalreplicas. Examples of suitable block copolymer compositions are providedby S. Förster and M. Antonietti in Advanced Materials 10, 195 (1998).Particularly preferred examples are diblock and triblock polymersinvolving linkages of either polystyrene, polybutadiene, polyisoprene,poly(methylacrylate), poly(propylene oxide), poly(dimethylsiloxane), orpolyethylene oxide.

Each of the templating process can be either a surface templatingprocess (by coating only a layer on the interior surface of the opals)or a volume templating process (by completely filling the void volume ofthe opals). Various methods can be used to obtain infiltration, such asmelt or solution infiltration, chemical vapor deposition (CVD)(including variants thereof like plasma-enhanced CVD), gas phasecondensation, electrochemical deposition, and reaction of an infiltratedfluid. In electrochemical infiltration it is preferable to attach theelectrochemical electrode to one side of an electrically insulatingopal, and to permit the electrolyte to penetrate from the opposite side,so that the electro-deposited material grows from the electrode coatedside of the opal to the opposite side. As examples of these processes,the deposition of a conducting polymer into the void volume of a porousopal is preferably by polymerization from a solution of thecorresponding monomer and a suitable oxidant, by electrochemicalpolymerization, by vapor phase infiltration (involving de-polymerizationand re-polymerization), by melt phase infiltration, or by depositionfrom a solution of the conducting polymer.

In a most preferred process for volume templating, the material to beinfiltrated is melted and then either wicked into the opal or forcedinto the opal by an applied pressure. In other most preferred processesfor volume templating, infiltration is by either CVD or gas phasecondensation. Volumetric infiltration of solids by electrochemicalprocesses, by in-situ reaction processes, and by chemical vapordeposition are preferred or most preferred only in special cases wherethe blockage of exterior channels before the complete filling ofinterior channels can be insured. This is the case when an infiltratedchemical either (a) reacts to form the nearly same volume of a desiredsolid or (b) this reaction proceeds from the inside to the exterior ofthe infiltrated sample in a bath of the infiltration fluid. This canalso be the case where a temperature or chemical gradient preferentiallycauses progressive and essentially complete reaction from interior voidspace to exterior void space. Likewise, uniform filling of an opal sheet(or inverse opal plate) by electrochemical reaction is most effectivelyaccomplished by choosing the opal or inverse opal sample to be made ofan insulating material, which is intimately connected on one plate sideby a conductor electrode. Examples of such suitable materials areinsulating inorganic glasses, ceramics, and polymers.

In two most preferred processes for surface templating, the surfacetemplating is by either the reaction of an infiltrated liquid (such asthe polymerization of polyaniline) or the deposition from an infiltratedliquid (such as by the evaporation of a solvent from an infiltratedpolymer solution). CVD and gas phase condensation are other mostpreferred processes for surface templating, since an initial surfacetemplating process can become a volume templating process when reactiontimes are increased, and this transition from surface templating tovolume templating can be prevented by keeping reaction times short.These surface templating processes are preferably accomplished asuniformly as possible by avoiding enhanced reaction at surfaces (thatresults in the clogging of the pore volume of the opal). For example, ifa CVD deposition (or other chemical reaction) is by the reaction of twogaseous species (or liquid phase species), such clogging of the porevolume can be accomplished for a flat opal sheet by introducing thesetwo species from opposite sides of the sheet.

Other templating processes of this invention are referred to a patchtemplating and as particle loading. Patch templating is a type oftemplating process where the surfaces of a void structure are coveredwith a partial surface coating of the infiltrated material (so thatuncoated regions exist). Depending upon the application need this patchcoating can be either percolated (called percolated patch) orunpercolated (called unpercolated patch). Patch coating is mostpreferably accomplished by inhomogeneous reaction from a solution, suchas in the deposition of a metal from a metal salt (like the depositionof Au from a solution of AuCl₄). Adjusting the reaction time can controlwhether or not the patch coating is percolated, since an unpercolatedpatch coating can become percolated upon further reaction. Particletemplating most preferably results from either the infiltration ofparticles into the opal (or the in-situ formation of such particleswithin said opal). The particles in particle-infiltrated opals arepreferably aggregated together to form a mechanically robust structure,thereby insuring that these particle do not de-aggregate duringextraction processes for the host matrix. For example, this aggregationcan be accomplished by post infiltration sintering. Infiltratedparticles are preferably smaller than ⅕th the diameter of the smallestinterconnections between void space in the infiltrated opal. Ultrasonicdispersion can be conveniently used to obtain particle infiltration froma colloidal dispersion. Alternately, or in conjunction withultrasonically assisted infiltration, particle infiltration can beaccomplished by passing a fluid containing the particles through a plateof the opal.

The void space in the face-centered-cubic opal structure is from about19 to about 25% for the sintered opals that are most preferred for theinvention embodiments. This means that the void space for a fullyvolume-templated opal will be about 25% or less. On the other hand,inverse opals obtained by-surface templating can have much higher voidvolumes. Void volumes from about 75% to nearly about 100% can beconveniently obtained using the processes of the present invention.

Three-dimensionally periodic structures of this invention can be dividedinto two categories: hierarchical crystals and non-hierarchicalcrystals. For example, a diamond inverse opal is a hierarchical crystal,since the cubic diamond structure on the atomic scale is unrelated tothe cubic lattice structure at optical wavelengths. On the other hand,one may use the processes of this invention to fabricate a form ofgraphite that is non-hierarchical (like a conventional crystalphase)—since it predominantly consists of graphite sheets whose localorientation and position crystallographically depends on an opal-derivedperiodicity (which is a thousand times longer than the graphiteinter-layer separation).

Depending upon the surface energy of an infiltrated fluid (such as aninfiltrated melt or liquid) and the dimensions of the interconnectionsbetween void space, it is sometimes necessary to apply pressure forinfiltration in the opal or inverse opal structures. High pressure isrequired for metal infiltration into the SiO₂ opals because of theabsence of significant wetting of the opal internal surface by theliquid metal phase. The lowest external pressure (P_(ext)) required forinfiltration can be calculated using the equation

 P _(ext)=(2σ_(s) cos Θ)/r,  (1)

where r is the capillary radius, σ_(s) is the surface tension of metalphase, and Θ is the contact angle between two surfaces (cos Θ=1 for thenon-wetting case). Thus, the smaller the minimum radius of theinterconnects between nearest octahedral and tetrahedral voids (shownschematically in FIG. 2), the higher the pressure P_(ext) that must beapplied for infiltration. The capillary radius depends on the diameterof the spheres and the degree of sintering. The minimum radius of thechannel between octahedral and tetrahedral voids can be calculated usingthe equation:

r _(min)=(3)^(−½) d−(D/2),  (2)

where d is an effective distance between silica spheres and D is theirinitial diameter. Complete isolation of the tetrahedral and octahedralvoids from each other occurs when the critical value of D/d=1.155 isreached. In this case, melt infiltration into the opal matrix isimpossible. Using a typical value of D/d=1.055, a sphere diameter of 220nm, and the surface tension of bismuth, the minimum pressure requiredfor infiltration of molten bismuth (calculated using Eqns. 1 and 2) isabout 0.7 kbar. The pressure that we use in order to obtain bismuthinfiltration into large samples of SiO₂ opal (6×6×6 mm³) in a reasonablyshort time is much higher (8-10 kbar at 300-350° C. for 1-2 hours).

It has been discovered (by scanning electron microscopic observations,differential scanning calorimetry measurements, and observation ofdramatic changes in conductivity) that remelting and resolidification(at ambient or low pressure) provides a method for engineering periodicarrays of breaks of 10 nm or smaller dimensions inside the materialsthat had been melt-infiltrated at high pressures (where such breaks donot exist). For the practice of this invention embodiment of opening upnanoscale gaps in infiltrated materials, it is preferred that theinfiltrated material has both a lower melting point than the confininghost lattice and a positive volume expansion upon melting. Also, it ispreferred that the initial melt phase infiltration (andresolidification) is conducted at a much higher pressure that thesubsequent melting and resolidification step. More preferably, thisinfiltration pressure is at least about 0.1 kbar. Finally, it ispreferred that the period of time that the infiltrated material is heldin the molten state prior to solidification (at low pressure) is keep asshort a time as convenient, since essentially complete de-infiltrationof the infiltrated material can occur (in some instances) if this periodis too long. The infiltrated material is preferably held in the moltenstate after remelting for no more than about 10 minutes. For thosematerial that easily de-infiltrate from the opal during low-pressuremelting and resolidification, one can optionally apply pressure duringthis process to prevent massive de-infiltration (which is preferablymuch smaller than the pressure used for the initial infiltration. It isbelieved that the origin of the observed effect is the positive volumeexpansion coefficient of the infiltrated material, which results inabout a few percent extrusion of the infiltrated material during thelow-pressure melting step. Upon recrystallization, there is notsufficient volume of infiltrated material to fill the void space of theopal (since a few percent of the initially infiltrated material has beenextruded). As a result, voids open up in the recrystallized infiltratedstructure (six voids per original sphere in the primary opal template),so as to minimize the unfavorable interfacial energy between theinfiltrated material and the spheres of the opal structure. This meansthat for an opal structure (based on 200 nm diameter spheres, whichcontains about 10¹⁴ spheres/cm³) a periodic array of very small gaps iscreated (down to a diameter of less than 10 nm) and extremely highdensity (about 6×10¹⁴ gaps/cm³). As will be described latter, suchability to engineer nanoscale gaps is very important for deviceapplications, such as thermionic materials. This process of nano-gapengineering is useful for a variety of materials, such as Sn, Pb, andTe-infiltrated SiO₂ opals. Evidence that such process is occurring isprovided by Differential Scanning Calorimetry, DSC (which shows that theoctahedral and tetrahedral void sites of the original opal latticeremain occupied after melting and subsequent resolidification), visualobservation of a limited amount of materials extrusion from the opalduring low pressure melting, and the occurrence of a transition frommetallic conductivity to extremely low room-temperature conductivity asa result this melting and resolidification process for infiltratedmetals (indicating that the metal within the opal is no longerpercolated).

Chemical reaction, melt phase de-infiltration, solution extraction,super-critical fluid extraction, and electrochemical dissolution aremost preferred methods for the extraction processes of this invention.Which method is utilized depends naturally upon the nature of thematerial being extracted and the material that should be unaffected bysuch extraction processes. The choice of the most suitable extractionprocess is conveniently determined by using the known characteristics ofthe materials in question, or by experiments that measure the rate atwhich components of opal derived structures are removed by particularchemicals. For example, the SiO₂ spheres of the SiO₂ opals can beextracted (or, if desired, partially extracted) by using either an acid(such as aqueous HF) or a base (such as alcoholic, aqueous ornon-aqueous sodium hydroxide or potassium hydroxide). Such extractioncan be either at ambient or lower temperatures, or at elevatedtemperatures to accelerate the extraction process. Such extractionprocesses can be accomplished at various steps in the creation ofcomplex three-dimensionally periodic structures. For example, it hasbeen found that extraction with aqueous KOH can be used to modify thestructure of the original opal in novel and useful ways. Limitedexposure to the base preferentially etches the sphere regions that areremoved from the inter-connect region for sintered SiO₂ opal—resultingin a template structure with an enhanced void volume, and dumbbell-likeinterconnections between neighboring spheres. Such a structure withrod-like struts between spheres has been unsuccessfully sought for thepurpose of making photonic bandgap crystals. Most preferably, theremoval of the material A is by treatment with either an acid, a base,or a solvating or reacting chemical at a temperature of below 200° C.Other less preferred methods can also be used for the extractionprocesses (or partial extraction processes), such as gas phase removalby sublimation, thermal degradation, and plasma reaction.

Surface-templated inverse opals of this invention are particularlyuseful for the fabrication of complicated structures comprised ofpotentially numerous components that are separately percolated to formcontinuous structures, as described below in more detail. Theseapproaches can utilize the existence of two separately percolated voidvolumes, which can be separately filled and separately electricallyaddressed. These void volumes correspond to (1) the space left by theextraction of the A spheres of the primary opal template (called S₁) and(2) the space exterior to both this volume and the material B (calledS₂, which is the unfilled portion of the original void volume of theprimary opal template). For example, described herein are processes formaking a redox display, supercapacitor, or battery using asurface-templated inverse opal. Using the processes of the invention onefirst makes a plate of surface-templated inverse from a material B thatis a solid-state electrolyte (which is an ionic conductor and anelectrical insulator). The opposite sides of this plate are referred toas the left and right sides. The electrically and ionically conductingredox materials (such as conducting polymers) that will be filled intothis inverse opal are called C₁ and C₂. Although the basic nature of thefollowing process does not depend upon this assumption, for simplicityit is assumed that C₁ and C₂ infiltrate (for example, from a fluidphase) more quickly into S₁ than S₂. The process is then as follows.

(a) Infiltrate C₁ from the left side of the plate, thereby initiallyfilling both S₁ and S₂ void spaces, and then (for further distances fromthe diffusion surface) only filling the S₁ void space. Infiltration ofthe S₁ void space is terminated immediately before this infiltrationreaches the right-hand side of the plate.

(b) The left side of the plate is etched away to the point that only theS₁ void space has been filled.

(c) The terminal ends of the infiltrated C₁ material (on the left-handsurface of the plate) are reacted so that they are permanently convertedto being an insulator.

(d) Infiltrate C₂ from the left side of the plate, allowing theinfiltration to proceed across the entire void space through S₂ (sincethe left-hand ends of the S₁ void space are already filled, thisinfiltration step does not effect that S₁ void space). Allow the C₂material to coat the left-hand side of the plate, thereby providing aneasily contacted electrode surface that is connected to all the C₂material.

(e) The terminal ends of the infiltrated C₂ material (on the right handsurface of the plate) are reacted so that they are permanently convertedto being an insulator.

(f) Infiltrate C₁ from the right side of the plate, allowing theinfiltration to proceed across the entire void space through S₁ (sincethe left-hand ends of the S₂ void space are already filled, thisinfiltration step does not effect that S₂ void space). Allow theinfiltrated C₁ material to coat the right hand side of the plate,thereby providing an easily contacted electrode surface that isconnected to all the C₁ material.

The consequence of this process, for which numerous variations areobvious from this teaching, is the formation of a redox displaymaterial, a supercapacitor, or a battery. If the original opal-derivedstructure was based on 250 nm spheres, and if the electrolyte coatingwas 40 Å thick, such a redox device would comprise about 10¹⁴/cm³ ofin-parallel redox devices that provide ultra-fast redox response becauseof the extremely high inter-electrode surface area and the nanoscaleinter-electrode separation. The following examples sectionexperimentally demonstrates how to make surface-templated inverse opalshaving these nanoscale-dimensions. Preferred materials made by thismethod, and by other related methods can be described as athree-dimensionally periodic material comprising at least threespatially separate compositions, wherein the interfaces between thesecompositions are replicas of the surfaces of a sphere array, wherein thesphere diameter is from about 20 nm to about 100 μm.

The processes of this invention may be used to synthesize novel anduseful forms of carbon having lattice parameter that are giant, so Bragodiffraction occurs at optical wavelengths and results in an opalescentappearance invention (Examples 5-8 and 10). Materials with such largelong periodicities (and significant refractive index fluctuations on thescale of these periodicities) are called photonic crystals. Like theshift from the dimensional scale and topologies of ordinary diamond andgraphite to those of the fullerenes, this further expansion to opticalwavelengths can provide new properties, such as a metallic photonicbandgap. The formation of carbon photonic crystals uses a templatingstep in which either carbon or a carbon precursor is infiltrated intoporous silica opal crystals. The silica crystals are sintered prior tothis infiltration, producing an extended interface betweennearest-neighbor spheres. This interface δ (shown in FIGS. 8 and 9)enables a final process, which is the extraction of the SiO₂ (using, forexample, aqueous HF or aqueous or alcoholic KOH). The resulting productis a carbon photonic crystal, which is called a carbon inverse opal(since the void space of the primary opal template is the only occupiedspace). Heretofore no one has been able to make either carbon phases orinsulator-free metals that are three-dimensionally periodic on the scaleof optical wavelengths.

The carbon infiltration inside the opal (either by CVD, plasma enhancedCDV, or by the infiltration of a precursor) either largely or completelyfills the void volume of the porous silica opal, thereby resulting(after silica extraction) in either a volumetric inverse opal or asurface inverse opal. The provided examples show the application of eachof these methods. In addition, there is a vast literature that describessuitable precursors, temperatures, and times that can be used for theformation of carbon phases. Some typical examples of suitablecompositions for CVD or plasma enhanced CVD are aromatics like benzeneand anthracene and alkanes like methane and ethane. Depending uponwhether the desired carbon product is graphitic or amorphous carbon,different precursors can be chosen for infiltration, and subsequentpyrolysis. For example, furfuryl alcohol reacts to form a resin thatdoes not readily graphitize. On the other hand, aromatic compositionssuch as poly(phenylene) readily graphitize. Both volumetric inverseopals surface inverse opals of carbon phases provide an assembly ofinterconnected tetrahedrally and octahedrally shaped motifs (with eighttetrahedrons and four octahedrons in the cubic unit cell), that arereplicas of the void space in the opals. These motifs are solid for anideal volumetric inverse opal, so the carbon phase comprise a singlelabyrinth. On the other hand, these motifs are hollow for the surfaceinverse opals, so there can be two separate three-dimensional labyrinths(one internal to the octahedral and tetrahedral motifs, called S₁, andthe other external, called S₂, as described above).

The fabrication of glassy carbon inverse opals uses another embodimentof the present invention, involving the infiltration of a carbonprecursor into the opal (and the subsequent polymerization andcarbonization of this precursor). This may be accomplished byinfiltrating millimeter-thick silica opal plates with an organic resin(such as a phenolic or furfuryl alcohol based resin), thermally curingthis resin at low temperatures, cutting the opal from the resin,removing residual surface phenolic by oxygen plasma etching, dissolvingthe SiO₂ from the resin infiltrated opal using aqueous HF, andpyrolizing the resulting resin inverse opal at progressively increasedtemperatures of up to about 1000° C. or higher (Example 6). Scanningelectron micrographs of fracture surfaces show a highly periodicstructure throughout the volume of both the phenolic and glassy carboninverse opals. The void structure comprises a face-centered-cubicarrangement of spherical voids, which are each interconnected withtwelve neighboring spherical voids through interconnects which can be assmall as a few hundred angstroms in diameter. As expected for a photoniccrystal with a large void volume, the phenolic and the glassy carboninverse opals are intensely opalescent. This opalescence shifts frompredominately red in the SiO₂ opal (based on 300 nm spheres) topredominately green in the phenolic inverse opal, and to darkblue/violet in the final carbon inverse opal (partly reflecting a 20% orhigher contraction in unit cell dimension upon the pyrolysis of thephenolic inverse opal). If the pyrolysis step occurs before theextraction of the SiO₂ spheres, the periodic wall structure of theglassy carbon inverse opal become porous (due to the volume changeassociated with pyrolysis), as shown in FIG. 3.

In another invention embodiment, carbon infiltration by chemical vapordeposition (CVD) is done to make graphitic inverse opals. Carboninfiltration into millimeter thick, and approximately 1-2 cm by 2-4 cmslabs of porous SiO₂ opal is accomplished using a 1:3 molar ratio ofpropylene and N₂ as the feed gas (one atmosphere for 6 hours at 800°C.), which is further described in Example 10. This procedure isfollowed by silica removal using aqueous HF. This CVD-produced carboninverse opal is opalescent and highly periodic and structurally regulardown to the 10 nm scale (as shown in FIG. 6). While x-ray diffractionmeasurements indicate that the phenolicderived carbon inverse opal is aglassy carbon, diffraction and C¹³ magic angle spinning spectra indicatethat the CVD-produced inverse opal is graphitic. Both the CVD andphenolic processes produced carbon inverse opals that are highlyconducting, providing a four-point-probe electrical conductivity ofabout 10 S/cm.

In another invention embodiment (described in Example 13),plasma-enhanced CVD has been used to make diamond inverse opals (shownat FIG. 7), as well as another type of carbon photonic crystal that isnon-hierarchical. Millimeter thick opal plates were seeded with 5 nmdiamond particles (which serve as nuclei for diamond growth) usingultrasonication of a suspension of 5 nm diamond grit in acetone in anultrasonic bath for 15 minutes. Carbon was subsequently deposited insidethe opal plates from a hydrogen/methane plasma created in a standardreactor by microwave excitation. Extraction of the SiO₂ spheres from thecarbon infiltrated opal (using aqueous HF) resulted in a material whosestructure depends upon the proximity to the exterior surface of theoriginal opal plate. About a 60 μm thick layer of cubic diamond(containing micron size crystallites with typical diamond crystalmorphology) exists on the exterior surface of the opal that was closestto the plasma. The carbon inverse opal closest to this surface is a 30μm thick layer arising from essentially complete filling of the porousopal with ordinary cubic diamond (identified by the electron diffractionpattern and a strong line at 1332 cm⁻¹ in the micro-Raman spectra).While poor adhesion is typically a problem in diamond coatingtechnology, the exterior diamond layer is intimately connected to thediamond inverse opal. Scanning electron micrographs show that thelattice of interconnected octahedral and tetrahedral voids of thestarting opal is replicated in the structure of the diamond inverse opal(see FIG. 7). The 300-500 μm thick layer immediately below the diamondinverse opal is a volumetric inverse opal (or a heavily filled surfaceinverse opal) consisting of graphitic carbon, which tends to separatefrom the diamond inverse opal layer during SiO₂ extraction. Theinnermost layer in the carbon deposited by the above described method(and Example 13) is composed of a new type of carbon phase (shown inFIG. 10), which we call a cubic graphite phase. In contrast with thecase for the carbon photonic crystals that we have previously described,the atomic arrangement of graphite sheets is determined by the cubicstructure existing at optical wavelengths. Hence the name cubicgraphite. Like proposed carbon phases called schwarzites, which wereoriginally based on the tiling of triply periodic minimal surfaces withsheets of sp² carbons [see. A. L. Mackay and H. Terrones, Nature 352,762 (1991)], the cubic graphite phase arises from tiling of the interiorsurface of the opal. However, unlike the proposed schwarzites, the cubicgraphite phase comprises graphitic sheets that are tiled upon eachother—reminiscent of the structure of carbon onions (which are carbonparticles consisting of concentric graphitic shells). The number of suchlayers depends upon deposition conditions and generally decreases withincreasing separation from the infiltration surface (within 200 μm thicksheets of surface inverse opal). However, a 40 Å layer thickness istypical for the innermost structure, and is used for discussions. Thevoid structure of the original opal is precisely replicated in thecarbon phase—indicating that these 40 Å thick stacks are sufficientlystrong to survive the stresses generated during the dissolution of theSiO₂ spheres (see FIG. 10 and Example 13). An electron diffractionpattern from cubic graphite provides diffraction spacings close to thatof ordinary graphite: 3.43 Å for (002), 2.10 Å for (100), 1.72 Å for(004), and 1.23 Å for (110). As expected because of the nested type ofstructure, the absence of reflections other than (hk0) or (001)indicates that the carbon layers are not in lateral registry. Covalentbonding between hollow spheres is indicated by the fracture mechanism ofthis and our other carbon inverse opals—fracture occurs across thesphere centers, rather than across intersphere necks. This is like theusual fracture mechanism of gem quality natural opal (but opposite tothat of the sintered porous opals).

In an idealized model for the cubic graphite phase, each graphite sheetextends throughout the entire lattice and is everywhere parallel to thevoid surface left by the dissolved SiO₂ spheres. Although structuraldefects modify this simplistic representation, the alignment of arraysof graphite sheets parallel to this void surface is evident fromtransmission electron micrographs. Ignoring the channels to neighboringspheres (typically), the sp² carbon tiled spheres in the carbon phaseare similar to a nested arrangement of giant fullerene spheres. Eachlayer of a nested stack in 250 nm spheres having 60 nm diameterinter-sphere channels contains about six million carbons. Consequently,the unit cell (which has the same Fm-3 m space group and latticeparameter as the original opal) contains about three hundred millioncarbon atoms in a nested 40 Å thick stack. No previously known processcan be employed to make a crystal structure (in which atomic positionsare strongly correlated with the lattice periodicity) that contains somany atoms.

The applications for these three-dimensionally periodic materials resultfrom this periodicity. One category of applications exploitsthree-dimensional structural periodicities that are in the visible,infrared, or ultraviolet regions to make optical switches, displaydevices, and directional light sources. In each case theperiodicity-dependent property being exploited is the Bragg scatteringof the electromagnetic radiation. This control on the direction ofpropagation of light (of a specified frequency) results from a change ofthe diffraction angle of this light because of a change in the unit cellparameter of the opal derived structure. This change in unit cellparameter can be conveniently accomplished by any of the variouswell-known methods that result in a change of materials dimension.Examples are the application of an electric field for anelectrostrictive or piezoelectric material; a temperature or pressurechange for either an ordinary material or a shape memory material;exposure to a solvent that causes swelling; the electrically-inducedchange in dimensions of a gel polymer; a thermally-inducedchemically-induced, or photo-induced reaction of a reactive matrixmaterial; and an electrochemically-induced dimensional change of a redoxmaterial (like carbon or a conducting polymer). Depending upon thechoice of materials and operating conditions, these changes can beeither reversible or irreversible, and can include the effects ofrefractive index change. The inverse opals synthesized by the presentprocesses are ideal for such optical switch applications, since thediffraction efficiency of an array of particle array is enhanced if thelow refractive index phase occupies a much larger volume fraction thanthe high refractive index phase. For a close-packed array of sphericalparticles (as in the porous SiO₂ opals), the maximum volume (about 76%)is occupied by the particles—so diffraction efficiency is not optimizedif the void space is air filled. However, in inverse opals made by theprocesses of this invention, an infiltrated material has filled the voidspace and the spherical particles (such as SiO₂) have been extracted(leaving air spheres). Hence, the low refractive index phase (i.e., theair phase) is now the majority phase, which is the situation thatmaximizes diffraction efficiency.

There are important applications for the three-dimensionally periodicstructures of this invention for electro-optical displays and switches.For example, in one invention embodiment flat plates of an inverse opalmay be made from an elastomeric material that is electrostrictive andopalescent. In Example 9 there is demonstrated the formation of a directopal of such an elastomeric material, by the reaction of a siliconeelastomer inside a polystyrene inverse opal, followed by dissolving thepolystyrene. As an alternative, an elastomeric polymer can beinfiltrated into an opal (such as a SiO₂ opal) from either a melt or asolution (or reactively formed with the opal), and the original opaltemplate can then be removed (such as by exposure to aqueous HF for thecase of the SiO₂ opal). If transparent non-constraining electrodes aredeposited on opposite sides of the flat plates and an electrical voltageis applied, the material will shrink, so as to minimize total energy.This electrically-induced shrinkage causes a color change because of ashift in the Bragg diffraction of the inverse opal. This color change isa new type of electrochromism that can be used for various types ofdisplays. Various methods can be used to enhance performance for thisinvention embodiment. For example, the elastomeric inverse opal can befluid filled (so as to increase dielectric breakdown strength) and theopal can be cut as single crystal sheet (so as to result in uniformityof the color change obtained for a particular viewing angle). This newelectrochromic effect can be combined with conventional technology fordisplays, so as to provide different colors from different pixels andviewing-angle-independent coloration.

Materials made by the processes of this invention are particularlyuseful as colorants for polymer article. With one exception, these novelcolorants can be co-mingled with conventional plastics by the samewell-known methods used for co-mingling conventional colorants withplastics. This exception is that the commingling process should not beso severe that small particle size and/or the destruction of latticeperiodicity adversely effect the coloration (by severe plasticdeformation). For this reason, it is preferred that the overall particlesize (corresponding to the crystal size if the particles are singlecrystals) exceeds at least 30 μm in at least one dimension. In contrastwith ordinary colorants, these colorants provide color throughdiffraction effects, rather than electronic absorption. Hence, thesecolorants can have extremely high stability to photodegradation andchemical degradation effects that fade conventional colorants. It ispreferable that these colorants for polymers have a temperature ofmelting or flow point that is in excess of the processing temperatureemployed for processing the matrix polymer and below the temperaturesthat result in substantial degradation of the matrix polymer. The reasonfor this preference is that such combinations of opal-derived colorantand matrix polymer provide the basis for an environmentally friendlymethod for achieving polymer coloration. Polymers are melt processes ata temperature where the colorant is stable (since it neither melts norflows). Later, during polymer recycling, the polymer may be reclaimed asa colorless material by using processing temperature that result inmelting or flow of the colorant materials, so as to destroy theperiodicity that is the origin of the coloration effect.

It has been observed that elastomeric materials (such as polyurethaneand silicone rubbers) made by the invention embodiments change colorwhen stretched. This effect arises because of the change in Braggdiffraction angle for a particular wavelength of light caused by themechanically-induced change in lattice parameters and refractive index.This unusual piezochromism is of practical importance both for theproviding an attractive cosmetic function to ordinary rubbers. Thispiezochromism can be used to provide a simple stress or strain sensors,which operate either because of a change in color or a change in opticalpathlength caused by the elastic distortions of the rubber lattice. Thispiezochromism is also useful for both optical switches and opticaldisplays, since conventional piezoelectric or electrostrictive materialscan be used to cause a color change or to deflect light beam in anoptical switch. The piezoelectric materials and electrostrictive thatare preferred for electro-mechanically driving are those providing atleast a 1% change in dimension in response to an applied voltage.Examples of such preferred materials are elastomeric rubbers and singlecrystal ferroelectrics known in the art. This dimensional change can beoptionally amplified by mechanical methods (such as by using variouscantilever-based methods well known in the art, like inch-worm, cymbal,moonie, and bimorph arrangements). It is more preferable that thepiezoelectric or electrostrictive materials (used for electro-mechanicaldriving of the opal-derived structures) provide at least a 5% change inlength in response to an electric field. Particularly preferredcompositions that provide such dimensional changes are elastomericrubbers.

Changing the refractive index of materials infiltrated into the inverseopals of our invention embodiments results in dramatic color changes ofthe inverse opals. The angle-dependent color of these materials reflectsa change in Bragg diffraction angle. This dependence of coloration onthe refractive index of the infiltrated material (or the refractiveindex of the inverse opal) provides a second mechanism for the operationof an optical switch, or a chromatic display element (which iscomplementary to the above-described mechanism of changing latticeparameter). This is shown in the examples where we compare chromaticeffects that we obtain for non-absorbing inverse opals (made frompolystyrene and poly(methyl methacrylate) with that for absorbinginverse opals (made from phenolic resin and carbon). For the purpose ofa switchable display element, we have also shown that incorporating anabsorbing material with switchable absorbency can be successfullyutilized. While it has been long known that refractive index changes ofmaterials infiltrated in ordinary direct opals can provide chromism, thedegree of such chromism is effected by the small filling factor of thesematerials (about 26 volume percent). In contrast, processes of thepresent invention provide void volumes from about 75% to nearly 100% forsurface-templated inverse opals, and this void volume can be filled byinfiltration. A void fraction of nearly 100 % results for the cubicgraphite phase of Example 13; a void volume fraction of about 74%results for the phenolic inverse opals of Example 1, the carbon inverseopals of Example 3, 6, and 7, the polystyrene inverse opal of Example 4,the poly(methyl methacrylate) inverse opal of Example 5, the epoxyinverse opal of Example 6, and the metal inverse opal of Example 17; anda void fraction of approximately 25% results for the elastomeric directopal of Example 9. The correspondingly larger accessible infiltrationvolume for our inverse opals provides a dramatically enhances colorationchange for a given change in refractive index of the infiltratedmaterial. Depending upon the application mode, this refractive indexchange can be a result of irradiation (for a radiation sensor), pressure(for a pressure sensor), applied electric field (for an electro-opticalswitch or electrochromic display element), or other variable of interest(such as a gas concentration or a magnetic field).

Structures having very high surface areas can be made by the processesof this invention, which leads to other applications—such as theelectrodes of batteries and supercapacitors, catalyst supports, gassensors, and storage materials for gases such as methane and hydrogen.The surface areas of the initial opals is not high (if the opals aremade of solid spheres) until very small sphere sizes are used, sinceeither the gravimetric or volumetric surface area is inverselyproportional to the lattice repeat length (and the sphere diameter). Forexample, opals based on 280 nm, 230 nm, and 190 nm diameter opalsprovide surface areas from mercury porosimetry measurements of 13.9,18.5, and 25.8 m²/gm, respectively. This surface area can bedramatically increased by going from a direct opal structure to aninverse opal structure, or by the activation of a material in an inverseopal to generate additional surface area. Even without activation,surface areas of 390 m²/gm (mercury porosimetry) can be obtained forinverse carbon opals (derived by a phenolic pyrolysis route/SiO₂extraction route described in this invention) based on an originaltemplate of 280 nm SiO₂ spheres. Electrically conducting forms of carbonare the most preferred composition for inverse opals for thisapplication. Because of the high conductivity of these carbons, and thehigh achievable surface areas, they are especially preferred were boththe electrical conductivity and high surface area are important, such aselectrodes for supercapacitors and high discharge rate batteries and asactuators based on the electrochemical double-layer effects (see R. H.Baughman in Synthetic Metals 78, 339 (1996) and B. K. Miremadi and K.Colbow, Sensors and Actuators B 46, 30 (1998) for methods for methodsfor using high surface area carbon in sensors and actuators).

Methods commonly employed for increasing the surface of ordinary formsof carbon (such as steam activation, carbon dioxide, and KOH activation)are also suitable for increasing the surface of carbon-based opals andinverse opals made by the processes of this invention. Such methods aredescribed by D. F. Quinn and J. A. MacDonald in Carbon 30, 1097 (1992),by P. N. Aukett et al. in Carbon 30, 913 (1992), and by R. Y. Lin and J.Ecomomy in Applied Polymer Symposium, No. 21, 143 (1973). The preferredaverage pore diameter in these carbons for gas storage applications isbetween 4 Å and 10 Å.

The extremely high surface areas and sharp structural featuresobtainable by the processes of this invention enable another importantapplication, which is an electron emissive element for devices such asdisplays and thermionic coolers and energy conversion units. For suchpurposes materials having work functions lower than about 2 eV are mostpreferred. An opal-derived structure that is most preferred for thisapplication is one that uses a cesium layer on a semiconductor, a lowwork function metal, a piezoelectric coating on a semiconductor,infiltration-deposited diamond doped diamond, wherein the shape anddimensional scale of these depositions replicates details of theoriginal opal template structure. For details on the specificcompositions and arrangements that have demonstrated utility for theseapplications (by lowering the surface barrier for electron emission) seeUnderwood et al. [Applied Physics Letters 73, 405 (1998) andreferences]. These same specific compositions and arrangements are thosethat are preferred for the present invention embodiments, where we takeadvantage of the unique structures provided by the inverse opals. Thesefeatures include the ability to make nanoscale breaks in theopal-derived structures, as well as the resulting ability to create atwo-dimensional array electron emitters having nanoscale sharpness forthe enhancement of electric fields. For example, the prior art has madenanoscale arrays of diamond tips electron emitters by using lithographicmethods. By the processes of this invention one may achieve arrays ofdiamond tips that are much less expensive to produce and much finer instructural detail. This is achieved by using a crystallographic plane ofthe diamond inverse opal as the emitter surface. A process for achievingsuch a structure is to crystallize a few monolayers of monodispersedSiO₂ spheres onto a conducting substrate, so that the most closelypacked plane is parallel to the substrate. Methods for achieving suchdeposition have been described by A. van Blaaderen et al. (Nature 385,321 (1997), as well as by earlier authors. Using the plasma-enhanced CVDprocess, diamond is deposited within the porous opal deposition. Ifnecessary, any excess diamond on the opal surface is etched away, suchas by using a hydrogen plasma. Subsequent etching of the SiO₂ spheres(such as with dilute HF or a strong base) will leave three pointedmicroelectrode elements surrounding each spherical hole on the surface,which corresponded to the original locations of a SiO₂ sphere. If theoriginal filling of the porous opal layer is terminated when the fillingreached the mid-point of the outermost layer of SiO₂ spheres, or ifsubsequent etching of the diamond occurs to this point, these diamondelectrode pillars would have a point diameter which approaches zero asD/d of Eqn. 2 approaches 1.155. Carbon forms that are especiallypreferred for this application are diamond with hydrogenated surfaces,polycrystalline diamond (where sp² carbons at grain boundaries conferelectrical conductivity), diamond-like carbon, and doped diamond(particularly nitrogen-doped carbon).

The use of the processes of the invention to make optical materialshaving photonic bandgaps is especially valuable, since photonic bandgapsmaterials are sought for applications because of a host of usefulproperties, such zero threshold lasing, and the ability to bend light atcurvatures as small as the wavelength of light (for optical circuit andoptical sensor applications). The inverse opals are also useful as adielectric-based photonic bandgap material, especially since a largevolume fraction of the low refractive index phase facilitates gapformation [See S. John, Phys. Rev. Lett. 58, 2486 (1987); S. John,Physics Today 44(5) 32 (1991); E. Yablonovitch, Phys. Rev. Lett. 58,2059 (1987); and E. Yablonovitch and K. M. Leung, Nature 391, 667(1998)]. In addition, the embodiments of the present process providemetallic inverse opals in which the combination of low filling factorand high in-plane conductivity (especially surface inverse opals, suchas our described cubic graphite phase) can provide a plasmon-definedphotonic bandgap in the infrared. Such a material (with aplasmon-defined photonic bandgap in the infrared) is of considerabletechnological interest, since prior-art limitations on the fabricationmethod have previously restricted such useful materials with metallicphotonic bandgap to the microwave region [see D. F. Sievenpiper, M. E.Sickmiller, and E. Yablonovitch, Phys. Rev. Lett. 76, 2480 (1996) and J.Joannopoulos, R. Meade, and J. Winn, “Photonic Crystals” (PrincetonPress, Princeton, N.J., 1995)].

The processes of this invention for obtaining three-dimensionallyperiodic structures by the templating and extraction processes forself-assembled opals also provide useful materials for chromaticseparations. The reasons for this utility include (a) the existence ofwell-defined channels having accurately defined dimensions, (b) theability to vary channel dimensions, and (c) the applicability of thepresent processes for making porous structures for virtually anymaterial (including carbons that survive in inert atmosphere to farabove 2000° C.).

The processes of the present invention can provide mechanically robustfoams having extremely tow densities, these processes are applicable forthe formation of low dielectric constant layers for high-densityelectronic circuits. Surface-templated inverse opal structures are mostpreferred for this application, because of the low achievable density,which translates into a very low achievable dielectric constant. Themethods of this invention can be used to introduce porosity in theinternal surfaces of such inverse opals porous, thereby further reducingthe dielectric constant. In a specially preferred embodiment, sphericalparticles are crystallized on the electronic substrate to form a porouslayer, a foam-forming organosilane is infiltrated into the opalstructure, and then polymerization (and carrier extraction) processesare conducted to create a foam within the interstitial space of theoriginal porous opal. In this preferred process the opal particles mustbe removable by a solubilization or degradation process (such ashydrogen plasma etching of carbon spheres), which does not effect thefoam formed by the organosilane. This extraction process is the finalstep in the formation of the low dielectric constant coating. In orderto obtain a porous structure that does not have a structuralnon-uniformity on a scale that effects deposited circuit elements, it ispreferable that the sphere diameter of the original opal does not exceedabout 100 nm. It is more preferable that this sphere diameter does notexceed about 50 nm. In another preferred process, the methods of thisinvention produce an inverse opal of an easily removed material (such asan easily dissolved polymer, such as polystyrene). The surfaces (orentire void volume) of this inverse opal are coated with a foam, and theinverse opal template is then removed—thereby providing the lowdielectric constant substrate material.

Since the processes of this invention provide materials having highinterfacial area and periodic nanostructure, these processes are usefulfor the formation of nanostructured thermoelectrics. Thesenanostructured thermoelectrics are most preferably obtained by theinfiltration of conventional thermoelectric materials (see sectionbelow) into opal, inverse opal and other opal-derived structures from amelt under pressure of 0.0-10.0 kbar at isostatic conditions or bychemical vapor deposition methods. The concept utilized here is thatscattering processes at the interface between opal and infiltratedthermoelectric material increase the thermoelectric figure of merit (ZT)by having a greater effect on phonon-mediated (lattice) thermalconductivity than on electronic conductivity. This enhancement of ZTfollows from such changes, since ZT=S²σT/(K_(l)+K_(e)), where S is theSeebeck coefficient, σ is the electrical conductivity, and K_(l) andK_(e) are the phonon and electronic components of thermal conductivity.This approach results in an increase of ZT for a prototype system:bismuth infiltrated into porous SiO₂ opal. A larger fractional decreasein thermal conductivity is found than for electrical conductivity(relative to bulk polycrystalline Bi). Since the thermopower is littlechanged, the overall effect observed is as much as a two-fold increaseof ZT compared with that for polycrystalline bulk bismuth. Previous workhas shown that ZT can be enhanced for granular systems, [L. D. Hicks andM. Dresselhaus, Phys. Rev. B47, 12727 (1993) and L. D. Hicks et al.,Appl. Phys. Lett. 63, 3230 (1993)]. However, while observations showthat ZT increases with decreasing grain size d below 3 μm, it declinesfor smaller particle sizes (due to a disproportionately large decreaseof σ caused by small-particle grain boundaries) [see D. M. Rowe in “CRCHandbook of thermoelectrics” ed. D. M. Rowe, CRC Press, Florida, 1995,pp 43-53]. The nanostructured materials of this invention, having aperiodic structure with continuous pathways and regular structuralfeatures, can avoid this problem at small dimensional scales. Theexamples demonstrate a process of this invention in which chemical vapordeposition of SiH₄ results in a coating of silicon on carbon inverseopals. In this case, exposure to a hydrogen plasma removes the carbon,thereby providing a silicon-based inverse opal thermoelectric.Replacement of SiH₄ with a SiH₄/GeH₄ mixture (preferable 5:1 ratio) willresult in deposition of Si_(0.8)Ge_(0.2) coating on the inner surfacesof opal. The lower thermal conductivity of Si_(0.8)Ge_(0.2) alloy,compared with pure Si, would improve the figure of merit of the system.

Various polymeric compositions are preferred as infiltrated materialsfor the purpose of invention embodiments, especially those in which theuse is an optical application. Examples of these compositions and therange of their refractive indices (when unorientated) at 589 nm are asfollows: polyolefins (1.47-1.52), polystyrenes (1.59-161),polyfluoro-olefins (1.35-1.42), non-aromatic non-halogenated polyvinyls(1.45-1.52), polyacrylates (1.47-1.48), polymethacrylates (1.46-1.57),polydienes (1.51-1.56), polyoxides (1.45-1.51), polyamides (1.47-1.58),and polycarbonates (1.57-1.65). Especially preferred polymers foroptical applications are those that have little light scattering in thevisible due to imperfections, such as polymers that are either amorphousor have crystallite sizes that are much smaller than the wavelength ofvisible light. Phenolic-based resins and furfuryl-alcohol based resinsare also especially preferred for invention embodiments.

Phenolic derived polymers are specially preferred for the fabrication ofinverse opal colorants for plastics and the creation of carbon inverseopals by the pyrolysis of organic polymers. Silicon containingacetylenic polymers [such aspoly[(phenylsilylene)ethynylene-1,3-phenylene-ethynylene] described byM. Itoh et al. in Advanced Materials 9, 1187 (1997)] are especiallypreferred for the formation of polymeric inverse opals that can bepyrolized with little weight loss. The reason for this preference isthis low weight loss, and the extremely high stability of the resultinginverse opal product comprising carbon reacted with silicon (which isstable in air when heated red hot).

Various ferroelectric polymers are preferred as infiltrated materialsfor the purpose of invention embodiments. The term ferroelectric polymeras used herein includes both homopolymers and all categories ofcopolymers, such as random copolymers and various types of blockcopolymers. This term also includes various physical and chemicalmixtures of polymers. Poly(vinylidene fluoride) copolymers, such aspoly(vinylidene fluoride-trifluoroethylene), P(VDF-TrFE), are especiallypreferred ferroelectric polymer compositions. Additional copolymers ofvinylidene fluoride that are especially preferred as infiltratedmaterials for the three-dimensionally periodic structures of the presentinvention are described by Tournut in Macromolecular Symposium 82, pp.99-109 (1994). Other especially preferred ferroelectric polymercompositions are the copolymers of vinylidene cyanide and vinyl acetate(especially the equal mole ratio copolymer) and odd nylons, such asnylon 11, nylon 9, nylon 7, nylon 5, nylon 3 and copolymers thereof.

Various conducting polymers in either doped or undoped forms arepreferred infiltration materials for embodiments of this invention. Themost preferred conducting polymers are polyaniline, polythiophene,polypyrrole, poly(phenylene vinylene), poly(phenylene), and copolymersand substituted polymers involving chain backbone segments that aresegments of the former polymers.

Various inorganic compositions are preferred as infiltrated materialsfor the purpose of invention embodiments, especially those in which theuse is an optical application. Examples of these compositions and therange of their refractive indices at 589 nm are as follows: 1) metaloxides such as titanium dioxide, zinc oxide, silica, zirconium oxide,and alumina; 2) carbon phases such as diamond (n about 2.42), glassycarbon, graphite, Lonsdaleite, and diamond-like carbon; 3) other highrefractive index inorganics such as bismuth oxychloride (BiOCl), bariumtitanate (n_(o) between 2.543 and 2.339 and n_(e) between 2.644 and2.392 for wavelengths between 420 and 670 nm), potassium lithium niobate(n_(o) between 2.326 and 2.208 and n_(e) between 2.197 and 2.112 forwavelengths between 532 and 1064 nm), lithium niobate (n_(o) between2.304 and 2.124 and n_(e) between 2.414 and 2.202 for wavelengthsbetween 420 and 2000 nm), lithium tantalate (n_(o) between 2.242 and2.112 and n_(e) between 2.247 and 2.117 for wavelengths between 450 and1800 nm), proustite (n_(o) between 2.739 and 2.542 and n_(e) between3.019 and 2.765 for wavelengths between 633 and 1709 nm), zinc oxide(n_(o) between 2.106 and 1.923 and n_(e) between 2.123 and 1.937 forwavelengths between 450 and 1800 nm), alpha-zinc sulfide (n_(o) between2.705 and 2.285 and n_(e) between 2.709 and 2.288 for wavelengthsbetween 360 and 1400 nm), and beta-zinc sulfide (n_(o) between 2.471 and2.265 for wavelengths between 450 and 2000 nm). As is conventional,n_(o) and n_(e) in the above list of refractive indices denote theordinary and extraordinary refractive indices, respectively, forcrystals that are optically anisotropic. The n_(o) refractive index isfor light propagating down the principal axis, so there is no doublerefraction, and the ne refractive index is for light having apolarization that is along the principal axis.

Ferroelectric ceramics (such as barium titanate and solid solutions ofBaTiO₃ with either SrTiO₃, PbTiO₃, BaSnO₃, CaTiO₃, or BaZrO₃) arepreferred compositions for the opal derived compositions of the presentinvention embodiments. Ceramics that are relaxor ferroelectrics are alsopreferred ferroelectrics for invention embodiments. Relaxorferroelectrics that are especially preferred for the present inventionhave the lead titanate type of structure (PbTiO₃) and disorder on eitherthe Pb-type of sites (called A sites) or the Ti-type of sites (called Bsites). Examples of such relaxor ferroelectrics having B sitecompositional disorder are Pb(Mg_(1/3)Nb₂/₃)O₃ (called PMN),Pb(Zn_(1/3)Nb_(2/3))O₃ (called PZN), Pb(Ni_(1/3)N_(2/3))O₃ (called PNN),Pb(Sc_(1/2)Ta_(1/2))O₃ , Pb(Sc_(1/2)Nb_(1/2))O₃ (called PSN),Pb(Fe_(1/2)Nb_(1/2))O₃ (called PFN), and Pb(Fe_(1/2)Ta_(1/2))O₃. Furtherexamples of relaxor ferroelectrics with B-site disorder are solidsolutions of the above compositions, such as(1−x)Pb(Mg_(1/3)Nb_(2/3))O₃-xPbTiO₃ and(1−x)Pb(Zn_(1/3)Nb_(2/3))O₃-XPbTiO₃. Another more complicated relaxorferroelectric that is preferred for the present invention is Pb_(1−s)²⁺La_(x) ³⁺(Zr_(y)Ti_(z))_(1−x/4)O₃, which is called PLZT. PZT (leadzirconate titanate, PbZr_(1−x)Ti_(x)O₃) is an especially preferredferroelectric ceramic for invention embodiments. PMN (lead magnesiumniobate, Pb(Mg_(1/3)Nb_(2/3))O₃) is another especially preferredmaterial, which becomes ferroelectric below room temperature. Ceramiccompositions obtained by the addition of up to 35 mole percent PbTiO₃(PT) to PMN are also especially preferred, since the addition of PT toPMN provides a method for varying properties (such as increasing theCurie transition temperature and varying the refractive indices) andsince a relaxor ferroelectric state is obtainable using up to 35 molepercent of added (i.e., alloyed) PT. Ceramic compositions that undergo afield-induced phase transition from the antiferroelectric to theferroelectric state are also preferred for obtaining composites thatundergo electric-field-induced switching of coloration. One preferredfamily is the Pb_(0.97)La_(0.02)(Zr, Ti, Sn)O₃ family that has beenfound by Brooks et al. (Journal of Applied Physics 75, pp. 1699-1704(1994)) to undergo the antiferroelectric to ferroelectric transition atfields as low as 0.027 MV/cm. Another family of such compositions islead zirconate-based antiferroelectrics that have been described by Ohet al. in “Piezoelectricity in the Field-Induced Ferroelectric Phase ofLead Zirconate-Based Antiferroelectrics”, J. American Ceramics Society75, pp. 795-799 (1992) and by Furuta et al. in “Shape Memory Ceramicsand Their Applications to Latching Relays”, Sensors and Materials 3,4,pp. 205-215 (1992).

For applications in which reversible color changes in response totemperature changes are desired, ceramics that undergo reversibleelectronic phase changes are particularly preferred as infiltratedmaterials for the present invention. Such compositions that undergoreversible transitions to highly conducting states upon increasingtemperature are VO₂, V₂O₃, NiS, NbO₂, FeSi₂, Fe₃O₄, NbO_(2,) Ti₂O₃,Ti₄O₇, Ti₅O₉, and V_(1−x)M_(x)O₂, where M is a dopant that decreases thetransition temperature from that of VO₂ (such as W, Mo, Ta, or Nb) andwhere x is much smaller than unity. VO₂ is an especially preferredcolor-changing particle additive, since it undergoes dramatic changes inboth the real and imaginary components of refractive index at aparticularly convenient temperature (about 68° C.). The synthesis andelectronic properties of these inorganic phases are described by Specket al. in Thin Solid Films 165, 317-322 (1988) and by Jorgenson and Leein Solar Energy Materials 14, 205-214 (1986).

Preferred thermoelectrics for making thermoelectric opal-derivedcomposites by the methods of the present invention embodiments are asfollows: bismuth, bismuth-tellurides (Bi₂Te₃-based alloys), bismuthantimonides (Bi_(1−x)Sb_(x), alloys with 0.02<x<0.13), and Bi₂Te₃alloyed with Sb₂Te₃. Especially preferred thermoelectrics for thisapplication are Bi₂₅Sb₆₈Te₁₄₂Se₆ (with ZT=0.96, p-type),Bi_(0.5)Sb_(1.5)Te_(3.13) (with ZT=0.9, p-type), Bi_(1.75)Sb_(0.25)Te_(3.13) (with ZT=0.66, n-type), (Sb₂Te₃)₅(Bi₂Te₃)₉₀(Sb₂Se₃)₅(with ZT=0.96, n-type). The infiltration of these alloys intoopal-derived structures of this invention is preferably accomplished bymelt infiltration at the lowest pressure compatible with infiltration(to decrease the tendency that we observe for the phases todisproportionate) and by chemical vapor deposition. These bismuth,tellurium, and bismuth alloys are preferably used for opal-derivedthermoelectrics operating at either close to ambient or at lowertemperatures. Preferred examples of compositions for opal-derivedthermoelectrics that operate at higher temperatures (about 700 to above1000 K) are PbS, PbSe, and PbTe and silicon-germanium alloys (Si—Ge). Anespecially preferred composition is Si_(0.7)Ge_(0.3). Other preferredthermoelectric compositions for infiltration into the opal-derivedstructures of the present invention embodiments are the skutterudites,for which CoSb₃, IrAs₃, RhP₃ are the simplest examples andLaCo_(3−x)Fe_(x)Sb₁₂ is a more complex example.

Photopolymerizable monomers, photo-dopable polymers, photo-degradablepolymers, and photo cross-linkable polymers are preferred for aninvention embodiment in which patterned exposure to actinic radiation(in the gamma ray through visible frequency range) is used for obtaininginverse opals having (a) arbitrary shape or patterning or (b) apatterned distribution in properties for the infiltrated material (suchas a desired distribution in refractive index). For example, a patternedthin layer deposition of inverse opal can be obtained on a substrate by(a) growing thin SiO₂ opal sheets on a substrate, (b) annealing thesesheets so as to obtain the inter-sphere interconnections that facilitatesphere extraction, (c) infiltrating the SiO₂ sphere sheets with aphotoresist, (d) irradiating this photoresist in a patterned manner withactinic radiation (for example, using a patterned mask), and (e)extracting both the SiO₂ spheres and either the irradiated onnon-irradiated photoresist. The resulting patterned deposition ofinverse opal can then be either used in this form for applications (suchas optical circuitry) or subjected to further infiltration/extractionsteps to provide patterned depositions of other opal derived materials.Materials suitable for this use are described, for example, in Chapter 1(pages 1-32) written by J. E. Lai in the book entitled “Polymers forElectronic Applications”, which is also edited by the same author (CRCPress, Boco Raton Fla., 1989). Improved materials that are now beingintroduced are described by G. M. Wallraff et al. in CHEMTECH, pp.22-30, April 1993. More exotic compositions suitable for the presentapplication are described by M. S. A. Abdou, G. A. Diaz-Guijada, M. IArroyo, and S. Holdcroft in Chem. Mater. 3, pp. 1003-1006 (1991).

The following examples are presented to more particularly illustrate theinvention, and should not be construed as being limitations on the scopeof the invention.

EXAMPLE 1

This example demonstrates the fabrication of a phenolic inverse opal bythe templating of a sintered porous opal that has a periodic structureat optical wavelengths. FurCarb® Resin (187 g, LP-520, which is afurfuryl-based phenolic resin available from Great Lakes ChemicalCorporation, P. O. Box 2200, West Lafayette, Ind. 47906) was vigorouslystirred for ten minutes at room temperature after adding four drops ofhydrochloric acid (37.5 wt %, Fisher) as the catalyst forpolymerization. A piece of sintered porous opal (FIG. 1) composed of 250nm SiO₂ spheres was placed into a small Teflon coated aluminum cupcontaining about 1.5 gm of the above resin containing HCl catalyst.After two days at room temperature, during which time the resininfiltrated the opal, the resin was cured in an oven for three days at80° C., two days at 100° C., and finally two days at 130° C. The curedresin was black. After removing the cured resin around theresin-infiltrated opal by grinding, this opal showed intenseopalescence. The surface of the infiltrated opal was cleaned for fiveminutes using a plasma cleaner. Thereafter, the weight of the phenolicinfiltrated opal was 49.7 mg. The SiO₂ spheres were removed from thissample by dissolution in hydrofluoric acid (48%) for three hours. TheSiO₂-free sample was then thoroughly washed with water and then driedover anhydrous CaSO₄ for one hour under vacuum. The resulting productwas an inverse phenolic inverse opal (weighing 7.1 mg) having the sameshape and size as the starting SiO₂ opal and showing brilliant colorsdepending upon the diffraction of light by the periodic inverse opalstructure. Scanning electron microscopy (SEM) investigation of afractured surface of this inverse opal indicates that all silica sphereswere dissolved by the hydrofluoric acid. The SEM picture revealed aperiodically arranged and interconnected structure that reliablyreplicated the void space in the original SiO₂ opal (FIG. 4). This newtype of material is called a polymer inverse opal, since a polymerstructure replicates the void space in the original opal.

EXAMPLE 2

This example demonstrates the intense diffraction-based coloration ofthe polymer inverse opal and the switching of coloration. A piece of thephenolic inverse opal was prepared according to the previous example. Itshowed a brownish light-blue color. This inverse opal was broken intofour pieces, which were placed in hexane, ethanol, acetone, and water,respectively. The sample in hexane showed a bright green color. The onein acetone gave red, green, and yellow opalescence depending on theorientation of the sample region with respect to the incident light. Theone in ethanol offered a light yellow-green color. The one in water didnot substantially change its color from that of the liquid-free inverseopal. These differences in coloration in the different liquids areattributed to the difference between the refractive indices of theseliquids (which are 1.375, 1.359, 1.357, and 1.333 for hexane, ethanol,acetone, and water, respectively).

EXAMPLE 3

The samples comprising cured phenolic resin in SiO₂ opal were preparedaccording to the procedure in Example 1. These samples were thenembedded in the powder of the cured resin and carbonized under argonusing the following thermal process. The sample temperature wasincreased from room temperature to 750° C. in five hours, maintained at750° C. for three hours, and then cooled down to room temperaturewithout temperature control. After the samples were treated with anoxygen-plasma for 5 minutes, they showed opalescence on a dark-blackbackground. Then the samples were further treated with hydrofluoric acidfor 2.5 hours, repeatedly washed with water, and dried over anhydrousCaSO₄. This overall process, used for removing the silica spheresdecreased the sample weight by about 87 to 90%, but did not cause achange in sample size and shape. This sample was opalescent and did notnoticeable change coloration when immersed in liquids having variousrefractive indices. An x-ray powder diffraction analysis indicates thatthe carbon in the inverse opal is amorphous. Also, SEM images show thatthe carbon inverse opal consists of periodical structure that is areplica of the void space in the original opal. Inverse carbon opalsprepared in this way have sufficient mechanical strength for use astemplates, which will be described in subsequent examples.

EXAMPLE 4

This example demonstrates the preparation of a polystyrene inverse opal(FIG. 11) using the in-situ polymerization of styrene inside the porousopal. This example shows that the glass transition of the polystyrene isdecreased (compared with that of bulk polystyrene) as a result of thesmall dimensional scale of the structural elements of this polystyreneinverse opal. A piece of synthetic opal was placed in styrene monomer(Aldrich, 99+%) containing 1.0 wt % 2,2′-azobisisobutyronitrile(Aldrich, 98%) for four hours under vacuum at room temperature. Thistreatment allowed the styrene and initiator to infiltrate the opal.Polymerization of the polystyrene in the opal was carried out at 60-80°C. for 24 hours. After removing the extra polystyrene around the opal bygrinding, the polystyrene-infiltrated opal was immersed intohydrofluoric acid (Aldrich, 48 wt % in water) for about one hour todissolve the SiO₂ spheres. Residual hydrofluoric acid was removed fromthe resulting inverse opal by repeated washing with water. Thispolystyrene inverse opal showed colorful opalescence and was composed ofa three-dimensional array of interconnected holes in a polystyrenematrix. The polystyrene inverse opal dissolved completely in FurCarb®520 at 100° C. and was deformed and partially dissolved in chloroform.The glass transition temperature of the polystyrene inverse opal wasmeasured by differential scanning calorimetry (10° C./min) and found tobe 85.52° C, which is 6° C. higher than that measured by the same methodfor bulk polystyrene. This result indicates that one can modify polymerthermal properties by the confinement in local dimension provided by thedimensional scale of the inverse opal lattice.

EXAMPLE 5

This example demonstrates the fabrication of a poly(methyl methacrylate)(PMMA) inverse opal. A piece of the porous SiO₂ opal of the typedescribed in Example 1 was placed for four hours in methyl methacrylatemonomer (Aldrich, 99%) containing 1.0 wt % 2,2′-azobisisobutyrontrile(Aldrich, 98%). This exposure permitted the infiltration of the monomerinto the opal. The monomer was polymerized within the opal at 40-60° C.for 24 hours. After removing the excess polymer around the opal bygrinding, the filled opal was immersed in hydrofluoric acid (Aldrich, 48wt. % in water) to dissolve the SiO₂ spheres. After repeated washings inwater and drying in air, the PMMA inverse opal was obtained as acontaminant-free product. This PMMA inverse opal was composed of athree-dimensionally periodic array of air-filled, spherical voids.

EXAMPLE 6

This example demonstrates the formation of an inverse opal based onanother epoxy composition, and then the conversion of this epoxy into aninverse opal of a hard carbon. The porous opal used as primary opaltemplate was the same as for Example 1, and the preparation method wassimilar to that of Example 1. The porous silica opal was filled by amixture of 30 ml Epox® 812 resin, 24.5 ml nadic methyl anhydride, and1.1 ml tris-dimethylaminomethyl phenol (Ernest F. Fullam Inc.), and thenreacted by thermal annealing for 24 hours at 60° C. The SiO₂ wasextracted from the phenolic-filled opal using HF (Aldrich, 48 wt. % inwater), and the sample was then repeatedly washed and dried, asdescribed in Example 1. The resulting phenolic inverse opal waspyrolized under argon at 900° C. for one hour. The resulting carboninverse opal was opalescence, and found by SEM to be athree-dimensionally periodic array of interconnected, nearly sphericalholes. This process also works to provide a carbon inverse opal when theabove mixture is replaced with a mixture of 30 ml Epox® 812 resin, 44.8ml dodecyl succinic anhydride, and 1.5 ml tris-dimethylaminomethylphenol (Ernest F. Fullam Inc.)

EXAMPLE 7

This example describes two different methods for forming a hard carboninverse opal. A porous SiO₂ opal was infiltrated over a two day periodat room temperature with a mixture of 187 g FurCarb® 520 resin and fourdrops concentrated hydrochloric acid (Aldrich, 37 wt. % in water).Polymerization of the resin in the silica opal was accomplished bythermal annealing at 80° C. for two days, at 100° C. for two days, andfinally at 150° C. for two days. Two samples of this FurCarb® filledopal were treated using two different procedures: (i) the first samplewas pyrolized at 900° C. under argon for one hour and then immersed in ahydrofluoric acid solution (Aldrich, 48 wt. % in water) in order toremove the silica spheres and (ii) the second sample was immersed in ahydrofluoric acid solution (Aldrich, 48 wt. % in water) in order toremove the silica spheres, and then pyrolized at 900° C. under argon forone hour. In both cases the resulting samples were FurCarb® inverseopals consisting of a periodic array of interconnected hollow spheres.Both samples showed opalescence resulting from this structuralperiodicity. However, the carbon sample that was pyrolized beforeextraction of the silica (see FIG. 5) had porosity resulting from thepyrolysis process, which was substantially absent for the sample thatwas pyrolized after extraction of the silica. SEM before and after thepyrolysis of the phenolic resin indicates a 25% contraction for thesample that had been extracted using HF before pyrolysis. Correspondingin part to this contraction (which was prevented in the sample that ispyrolized before extraction of the SiO₂ spheres) the color of thecarbonized phenolic is blue (while the unpyrolized phenolic is green).

EXAMPLE 8

This example demonstrates the application of the FurCarb® inverse opalprepared in Example 1 for the preparation of three-dimensionallyperiodic inverse opal filled with polystyrene. This opal is a periodiccomposite of phenolic and polystyrene which is periodic at opticalwavelengths. A piece of the FurCarb® inverse opal was placed for fourhours in a solution of a 100:1 weight ratio of styrene monomer (Aldrich,99+%) and 2,2′-azobisisobutyrontrile (Aldrich, 98%)—which enabled theinfiltration of the inverse opal with the styrene. The styrene was thenpolymerized at 60-80° C. for 24 hours. The product was ground into thesize of the original opal replica, so as to remove excess polystyrenefrom around the inverse opal. The resulting product (showingpredominately green opalescence) was shown by SEM to be athree-dimensionally periodic arrangement of interconnected polystyrenespheres in a three-dimensionally periodic FurCarb® matrix.

EXAMPLE 9

This example demonstrates the formation of an elastomeric opal based ona silicone elastomer. The inverse polystyrene inverse opal made by theprocess of Example 4 was infiltrated with a mixture of Sylgard® 184 andSylgard® 184 curing agent (10:1 by weight) and cured at room temperaturefor 48 hours. The inverse opal filled with the silicone elastomer wasground into the size of the original polystyrene inverse opal and thenimmersed in toluene to remove the polystyrene matrix. The resulting opalconsisting of only of the silicone elastomer showed strong opalescencewhen immersed in toluene.

EXAMPLE 10

This example demonstrates the formation of carbon inverse opals by achemical deposition (CVD) process in which carbon is formed on a porousopal, and the silica is removed by treatment with aqueous HF. For afirst sample, the carbon was coated on the internal surface of a opal(300 nm diameter spheres) by passing a one atmosphere mixture of 25%propylene (C₃H₆) and 75% nitrogen through a heated quartz tube (800° C.)in which the opal specimen (2 cm width, 4 cm length, and 2 mm thickness)was placed. The CVD time was twelve hours. For a second sample, thecarbon was coated on the internal surface of a opal (220 nm diameterspheres) by passing the above gas mixture through a heated quartz tube(800° C.) in which the opal specimen (2 cm width, 5 cm length, and 1 mmthickness) was placed. The CVD time was six hours. For a third sample,the carbon was coated on the internal surface of a opal (consisting of160 nm diameter spheres) by passing a one atmosphere of propylene gasthrough a heated quartz tube (900° C.) in which the opal specimen (2 cmwidth, 4 cm length, and 1 mm thickness) was placed. The CVD time was sixhours. Each of the above samples was placed in concentrated aqueous HFfor 12 hours, and then repeatedly washed with water and dried in air. Ineach case the product was an opalescent carbon inverse opal that SEMinvestigation showed was a three-dimensionally periodic carbon foam.Typical electrical conductivities measured for these foams (by the fourpoint probe method) were about 110-120 mΩ-cm. These values arecomparable to those that we obtained for the phenolic-derived glassycarbon inverse opals (typically 130-170 mΩ-cm).

EXAMPLE 11

This example demonstrates the application of FurCarb® inverse opalprepared in Example 1 for the preparation of three-dimensionallyperiodic inverse opal filled with poly(methyl methacrylate). This opalis a periodic composite of phenolic and poly(methyl methacrylate) whichis periodic at optical wavelengths. A piece of the FurCarb® inverse opalwas placed for four hours in a solution of a 100:1 weight ratio ofmethyl methacrylate monomer (Aldrich, 99+%) and2,2′-azobisisobutyrontrile (Aldrich, 98%)—which enabled the infiltrationof the inverse opal with the methyl methacrylate. The methylmethacrylate was then polymerized at 60-80° C. for 24 hours. The productwas ground into the size of the original opal replica, so as to removeexcess methyl methacrylate from around the inverse opal. The resultingproduct (showing predominately green/red opalescence) was shown by SEMto be a three-dimensionally periodic arrangement of interconnectedpoly(methyl methacrylate) spheres in a three-dimensionally periodicFurCarb® matrix.

EXAMPLE 12

This example demonstrates the preparation of a polystyrene/glassy-carboncomposite that consists of a phenolic-derived glassy-carbon inverse opalwhose void space is filled with polystyrene. The glassy-carbon inverseopal was made by the process of Example 3. A piece of the glassy-carboninverse opal was placed in styrene monomer (Aldrich, 99+%) containing1.0 wt % 2,2′-azobisisobutyronitrile (Aldrich, 98%) for four hours undervacuum at room temperature. This treatment allowed the styrene andinitiator to infiltrate the inverse opal. Polymerization of thepolystyrene in the inverse opal was carried out at 60-80° C. for 24hours. The electron micrograph of FIG. 12 shows that the resultingthree-dimensionally periodic composite contains a polystyrene directopal lattice that interpenetrates a glassy carbon inverse opal lattice.

EXAMPLE 13

This example demonstrates the preparation of sheets of diamond inverseopal, sheets of volumetrically-templated carbon inverse opal, and sheetsof surface-templated graphite inverse opal, called cubic graphite. Thisprocess was by plasma enhanced CVD of carbon in diamond particle seededSiO₂ opal, followed by the extraction of the SiO₂ spheres using aqueousHF. The diamond nuclei were seeded within millimeter opal plates byultrasonic agitation of these plates in an acetone suspension of 5 nmdiamond grit. A microwave-plasma-enhanced chemical-vapor-depositionreactor operating at 2.45 GHz and 3.5 kW [V. G. Ralchenko et al.,Diamond and Related Materials 6, 159 (1997)] was used to infiltratecarbon into opal plates (comprising 250 nm SiO₂ spheres) from a CH₄—H₂fed plasma (50-60 Torr pressure and gas flows of 972 sccm for H₂ and 25sccm for CH₄). The deposition substrate temperature reached by plasmaheating was 750-850° C. and the deposition time was 64 hours. Afterattaching the diamond surface of a carbon-infiltrated opal sheets to agraphite plate substrate (using colloidal graphite paint), the SiO₂spheres were removed using a one day exposure to 10% hydrofluoric acid,and the samples were repeatedly washed with water. The structure of theextracted material depends upon the proximity to the exterior surface ofthe original opal plate. About a 60 μm thick layer of cubic diamond(containing micron size crystallites with typical diamond crystalmorphology) exists on the exterior surface of the opal that was closestto the plasma. The carbon inverse opal closest to this surface is a 30μm thick layer arising from essentially complete filling of the porousopal with ordinary cubic diamond (identified by the electron diffractionpattern and a strong line at 1332 cm⁻¹ in the micro-Raman spectra).While poor adhesion is typically a problem in diamond coatingtechnology, the exterior diamond layer is intimately connected to thediamond inverse opal. Scanning electron micrographs show that thelattice of interconnected octahedral and tetrahedral voids of thestarting opal is replicated in the structure of the diamond inverse opal(FIG. 7). The 300-500 μm thick layer immediately below the diamondinverse opal is a volumetric inverse opal (or a heavily filled surfaceinverse opal) consisting of graphitic carbon, which tends to separatefrom the diamond inverse opal layer during SiO₂ extraction. Theinnermost layer in the deposited carbon is composed of a new type ofcarbon phase, which we call a cubic graphite phase (FIGS. 8 and 10). Incontrast with the case for the carbon photonic crystals that we havepreviously described, the atomic arrangement of graphite sheets isdetermined by the cubic structure existing at optical wavelengths. Hencethe name cubic graphite. This cubic graphite phase comprises graphiticsheets that are tiled upon each other—reminiscent of the structure ofcarbon onions (which are carbon particles consisting of concentricgraphitic shells). The number of such layers depends upon depositionconditions and generally decreases with increasing separation from theinfiltration surface (within 200 μm thick sheets of surface inverseopal). However, a 40 Å layer thickness is typical for the innermoststructure. The void structure of the original opal is preciselyreplicated in the carbon phase—indicating that these 40 Å thick stacksare sufficiently strong to survive the stresses generated during thedissolution of the SiO₂ spheres. An electron diffraction pattern fromcubic graphite provides diffraction spacings close to that of ordinarygraphite: 3.43 Å for (002), 2.10 Å for (100), 1.72 Å for (004), and 1.23Å for (110). As expected because of the nested type of structure, theabsence of reflections other than (hk0) or (001) indicates that thecarbon layers are not in lateral registry.

EXAMPLE 14

This example demonstrates the preparation of an inverse opal of aferroelectric polymer. A piece of the porous SiO₂ opal of the typedescribed in Example 1 was placed into a 50 wt % solution of (50/50)poly(vinylidene difluoride-co-trifluoroethylene) in cyclohexanone. After20 hours at 155° C., the cyclohexanone was removed under vacuum at 130°C. During this process, the copolymer continuously infiltrated into theopal as the polymer concentration increased. After removing the solvent,the copolymer-infiltrated opal was separated from the surroundingpolymer and its surfaces were polished. The infiltrated opal was brokeninto two pieces. One was examined by scanning electron microscope (SEM)and the other was treated in hydrofluoric acid (Aldrich, 48 wt. % inwater) to remove the SiO₂ spheres, and then repeatedly washed withwater. The SEM micrographs of the first piece show that the SiO₂ sphereswere surface coated by the copolymer. The HF treated sample showsintense opalescence, indicating that an inverse opal had been formedfrom a piezoelectric polymer.

EXAMPLE 15

This example demonstrates the infiltration of silicon into a porous SiO₂opal of Example 1, so as to provide closely spaced, non-percolatedsilicon particles infiltrated within the void space of this opal. Anopal sheet (having a thickness of 1 mm, a length of 60 mm, and a wide of20 mm) was placed into the CVD chamber and evacuated to 10⁻³ mtorr.Afterwards, the precursor gas composed of 97% nitrogen and 3% silane(SiH₄) was introduced into the CVD chamber at ambient temperature. Thisprecursor gas was delivered with the rate 300 cm³ per minute in order tomaintain the pressure 400 mtorr. The temperature was then increased upto 600° C. and held at this temperature for 5 hours. The resultingweight gain of the opal sample caused by the silicon infiltration wasabout 3% (a portion of which was in a very thin surface layer).

Examination of the fracture surface of this opal sheet showed that thesilicon has deposited throughout the opal sample (which was visuallyindicated by the red coloration throughout the sample thickness). SEMmicrographs shows that this deposited silicon is in the form of closelyspaced, non-percolated particles within the opal void space. Thissilicon infiltrated opal is useful for embodiments of the presentinvention where a low-work-function thermionic material is generated bysubsequent exposure of the silicon particle within the opal to cesiumgas, which creates a low-work-function cesium-coated silicon surface.For the purpose of this application mode, the silicon filing should beeither the present non-percolated particle type of filling or anon-percolated patch-type coating on the spheres. Application of theabove type of silicon CVD process to a surface-templated or avolumetrically-templated carbon inverse opal (or a porous direct opalmade of carbon spheres), so as to provide a uniform surface coating withsilicon (followed by the removal of the carbon, by low temperaturecombustion of the carbon or removal of the carbon by hydrogen plasma)will result in a silicon-based photonic bandgap crystal.

EXAMPLE 16

This example demonstrates the preparation of a metal/glassy-carboncomposite that consists of a phenolic-derived glassy-carbon inverse opalwhich is filled with an alloy. The glassy-carbon inverse opal was madeby the process described in Example 3. A piece of the glassy-carboninverse opal was placed in a capsule, which was tightly filled with apowder of Woods metal (Bi:50%, Pb:25%, Sn:12.5%, Cd:12.5% alloy) andhermetically sealed. The impregnation of the inverse opal with metaloccurred upon heating the capsule to the melting point of the powderunder small external pressure (0.1-3 kbar) for 1-30 minutes. Theimpregnation from the melt under isostatic conditions excluded anydistortions of periodic matrix of the inverse opal. Electron micrographsin FIG. 13 indicate that the three-dimensional structure of inverse opalmatrix was filled with Wood's metal. An accelerating voltage of 20 kVwas chosen for this micrograph so that the penetration depth of theelectron beam in carbon was longer than the wall thickness for thecarbon matrix. These walls are then semi-transparent to the electrons,which enables the underlying layer of ordered metal spheres to beclearly seen in FIG. 13.

EXAMPLE 17

This example demonstrates the preparation of an inverse opal made ofbismuth (FIG. 14). In a typical solid sample preparation, a rectangularpiece of opal (with average size 6×6×8 mm³) was surrounded with 200 meshBi powder (99.999% purity from Alfa Aesar) that was contained in a 9 mmdiameter stainless steel cylinder. Using a piston-cylinder pressurecell, this cylinder was held at 300-350° C. under a pressure of 8-10kbar for 1-2 hours. During this process, molten Bi infiltrated into theopal and filled the void space. After cooling under pressure and returnto ambient temperature, the opal samples were carefully cut from thesurrounding Bi matrix using a jeweler's saw and polished. The resultingBi-infiltrated opal samples had an average cross-section of 5×5 mm² anda thickness varying between 2 and 5 mm from sample to sample. TheBi-impregnated opal was placed in 48% hydrofluoric acid for 24 hours.Subsequently, it was washed with deionized water and acetone in an inertatmosphere and dried in vacuum. FIG. 14 shows that the acid dissolvedthe SiO₂ spheres in the system and did not react with the bismuth.

What is claimed is:
 1. A process for substantially eliminating thecoloration of a material comprising particles of a colorant in a matrixpolymer wherein the particles of the colorant comprise an array which isthree dimensionally periodic at visible wavelengths which processcomprises heating the material to a temperature that is higher than themelting temperature of the colorant, and below the degradationtemperature of the matrix polymer.
 2. A process for substantiallyeliminating the coloration of a material comprising particles of acolorant in a matrix polymer wherein the particles of the colorantcomprise an array which is three dimensionally periodic at visiblewavelengths which process comprises heating the material to atemperature that is higher than the melting temperature of the colorant,and below the degradation temperature of the matrix polymer, and furthercomprising the following steps for preparing said colorant: (a)crystallizing spheres of material A into a first structure havingthree-dimensional periodicity, and voids between spheres, wherein thematerial A is mechanically and thermally stable to at least about 300°C., (b) treating this first structure so that necks are formed betweenthe spheres of material A, (c) infiltrating said first structure withmaterial B to form a composite structure, and (d) removing material Afrom said composite structure to form a second structure comprisingmaterial B.
 3. The process according to claim 1, wherein said colorantcomprises a polymer.
 4. The process according to claim 3 wherein saidpolymer is selected from the group consisting of polyolefin,polystyrene, polyfluoro-olefin, non-aromatic non-halogenated polyvinyl,polyacrylate, polymethacrylate, polydiene, polyoxide, polyamide, andpolycarbonate.
 5. The process according to claim 4, wherein said polymeris selected from the group consisting of poly(vinylidene fluoride)copolymer; copolymer of vinylidene cyanide and vinyl acetate; nylon 11,nylon 9, nylon 7, nylon 5, nylon 3 and copolymers thereof.
 6. Theprocess of claim 2, wherein the first structure is either cubic,hexagonal, or a mixture of face-centered cubic and hexagonal packingarrangements; the spheres of material A are substantially monodispersed;and the three-dimensional periodicity of the first structure isreplicated in the said second structure.
 7. The process of claim 2,wherein the first structure comprises an array of SiO₂ spheres havingdiameters of from about 20 nm to about 100 μm and wherein theinfiltration is by a melt infiltration process that substantiallycompletely fills the space between spheres A.
 8. The process of claim 2,wherein the removal of the material A is by treatment with either anacid, a base, or a solvating or reacting chemical at a temperature ofabout 200° C. or below.
 9. The process of claim 2, wherein the firststructure is either body-centered-cubic or hexagonal close-packed andwherein the three-dimensional periodicity of the said first structure isreplicated in the said second structure.
 10. The process of claim 2,wherein the infiltration of said first structure with material B resultsin the filling of substantially the entire space between spheres A. 11.The process of claim 2, wherein the infiltration of said first structurewith material B results in the filling of less than about 10% of thespace between spheres A.
 12. The process of claim 2, wherein the spheresof material A are crystallized into a cubic or hexagonal sphere array,or mixture of cubic and hexagonal arrays, and wherein these spherearrays of material B are mechanically deformed to decrease symmetryeither prior to or after the infiltration step (c).
 13. The process ofclaim 2, wherein the step (a) crystallizing of the spheres of material Ais accomplished with the spheres on a substrate and the thickness of thecrystallized sphere array on the substrate is less than about 1 mm. 14.The process of claim 13 wherein steps (a)-(d) are accomplished on saidsubstrate and said substrate is substantially planar.
 15. The process ofclaim 14, wherein said substrate is patterned with a periodic array ofholes, troughs, or protuberances, and such results in acrystallographically oriented crystallization of the spheres of materialA.
 16. The process of claim 2 that additionally comprises infiltratingthe material obtained from step (d) with a material C to form a B-Ccomposite structure having three-dimensional periodicity.
 17. Theprocess of claim 16 wherein the material B in said B-C compositestructure is partially or substantially completely removed to form athree-dimensionally periodic structure comprising the material C. 18.The process of claim 2, wherein said first structure comprises an arrayof at least two or more sphere diameters, each of which is from a bout20 nm to about 100 μm.
 19. The process of claim 18, wherein said firststructure comprises a periodic array of two different diameters ofmonodispersed spheres, N and M, wherein this array has the compositionMN₁₃ or MN₂ and the radius of the N spheres are smaller than for the Mspheres.
 20. The process of claim 2, wherein the said first structureadditionally comprises cylinders of material A₁, having monodisperseddiameters, wherein this first structure in obtained by thecrystallization of said materials A and A₁.
 21. The process of claim 2wherein the first structure having three-dimensional periodicity istreated with a reagent that increases void volume, prior to theinfiltration of material B.
 22. The process of claim 1 wherein the saidcolorant is comprised of a block copolymer.
 23. The process of claim 2wherein the colorant is for ultraviolet, visible or infrared wavelengthsand the average sphere diameter is from about 20 nm to about 1 μm. 24.The process of claim 1 wherein the colorant is dispersed in amelt-processible matrix polymer, wherein the melting point or flow pointof the colorant is above the processing temperature of the matrixpolymer and lower than the temperature where the matrix polymerundergoes substantial thermal degradation, and wherein the particle sizeof the colorant dispersed in the matrix polymer is at least about 30 μmin at least one dimension.
 25. The process of claim 2 wherein the matrixpolymer is an elastomer and the material changes color when stretched.